Pixelmon is a Forge mod that...well, add Pokémon to Minecraft! It's a pretty cool mod, since I've used it before. They're also working on version 3.0, so be ready!
I don't know what you mean by "can't join Pixelmon", like if it was a server or something, so I guess I should clarify like this.
Now, I don't know how to help you with the Forge problem because I don't have much details, but I'll try helping ya out. Follow these steps:
Download the recommended Forge installerfor 1.6.4 (or 1.6.2) from their site (files.minecraftforge.net). The installer's the simplest and quickest way to get your Minecraft Forge running.
Next, right-click the installer file. Look for the option of "Open with...", then choose "Java(TM) Platform SE Binary".
A window should pop open on the screen. Choose "Install client", & then install it. As soon as you get the confirmation of the Forge being created, continue on.
Now you have to check your .minecraft folder. If you don't know how to find it, here's how. Do [Windows Key] + R. A "Run" window should appear. Type in "%appdata%" then Enter. You'll see a a File Explorer window with a bunch of folders in front of you. Go to the ".minecraft" folder. Then go to the "versions" folder. Then, you're ready to properly install Forge.
There's gonna be at least 2 folders here. This is where different versions of the game are stored. First things first, make a new folder & name it whatever you want (just remember the name first). You could name it "Forge", or "1.7.2", or "Apples". Anything's good to go.
Now, there should be an odd-named folder that has the word Forge in it. Enter that folder. In it, there's 2 files, a .jar file & a .json file. Copy those files, & paste them in your new folder that you just created. You can delete the original folder if you want.
OK, so far so good. Now, rename both files to the same name as the folder. If the folder is called "Diamonds to you!", rename both files to "Diamonds to you!" then.
Almost there! Here's a small tricky step. You're going to need to open the .json file with a text editor. Right-click it, then use "Open with...", and click "Choose default program...". Pick any text editor, like Notepad or Wordpad. Now, when you're in the .json file, scroll all the way down & find something like this: ["id": "X.X.X-ForgeX.XX.X.XXX"]. You are going to rename the [X.X.X-ForgeX.XX.X.XXX] with the same name as the folder.
Just one more step! You pretty much ready to go to play Forge. All it's left to do is to make a new profile in the Minecraft launcher, & then choose in "Use version:", choose "release [name of the folder]". If this is the first time using Forge, click Play, & let it run. This might take a while since it needs to create new folders and files. As soon as it's done...
I don't know what you mean by "can't join Pixelmon", like if it was a server or something, so I guess I should clarify like this.
The mod is found in this site: www.pixelmonmod.com
Now, I don't know how to help you with the Forge problem because I don't have much details, but I'll try helping ya out. Follow these steps:
- Download the recommended Forge installer for 1.6.4 (or 1.6.2) from their site (files.minecraftforge.net). The installer's the simplest and quickest way to get your Minecraft Forge running.
- Next, right-click the installer file. Look for the option of "Open with...", then choose "Java(TM) Platform SE Binary".
- A window should pop open on the screen. Choose "Install client", & then install it. As soon as you get the confirmation of the Forge being created, continue on.
- Now you have to check your .minecraft folder. If you don't know how to find it, here's how. Do [Windows Key] + R. A "Run" window should appear. Type in "%appdata%" then Enter. You'll see a a File Explorer window with a bunch of folders in front of you. Go to the ".minecraft" folder. Then go to the "versions" folder. Then, you're ready to properly install Forge.
- There's gonna be at least 2 folders here. This is where different versions of the game are stored. First things first, make a new folder & name it whatever you want (just remember the name first). You could name it "Forge", or "1.7.2", or "Apples". Anything's good to go.
- Now, there should be an odd-named folder that has the word Forge in it. Enter that folder. In it, there's 2 files, a .jar file & a .json file. Copy those files, & paste them in your new folder that you just created. You can delete the original folder if you want.
- OK, so far so good. Now, rename both files to the same name as the folder. If the folder is called "Diamonds to you!", rename both files to "Diamonds to you!" then.
- Almost there! Here's a small tricky step. You're going to need to open the .json file with a text editor. Right-click it, then use "Open with...", and click "Choose default program...". Pick any text editor, like Notepad or Wordpad. Now, when you're in the .json file, scroll all the way down & find something like this: ["id": "X.X.X-ForgeX.XX.X.XXX"]. You are going to rename the [X.X.X-ForgeX.XX.X.XXX] with the same name as the folder.
- Just one more step! You pretty much ready to go to play Forge. All it's left to do is to make a new profile in the Minecraft launcher, & then choose in "Use version:", choose "release [name of the folder]". If this is the first time using Forge, click Play, & let it run. This might take a while since it needs to create new folders and files. As soon as it's done...
- You're ready to use Forge!

Hope I helped!