Next up, open up the FTB launcher. Select Monster, and at the top there's a gray button that says Edit Modpack. From there, at the bottom there's a button that says 'Add Mod'. Clicking it will open a file browser. Navigate to your default Download directory (probably Downloads if you use Win7). You'll want to select the file called 'witchery-1.6.4_0.25.5'.
Once it's been selected and added to modpack, we need to do some editing on a Config file. Navigate to Monster > minecraft > config. In there, there's a file called 'witchery.cfg'. You'll want to open it with Notepad. What's happened is that Witchery tries to use some Item ID's that conflict with Engineer's Toolbox. I've gone through can changed these already, so you'll just need to copy/paste over the whole file. The config:
Save the .cfg and close Notepad. Now start up Monster, and you will now have Witchery working and installed!
I really like Witchery, as it has a flavor unique to itself. I'd actually like to see Witchery and Ars Magica 2 added to more main modpacks in the future, but for the time being I'll just keep adding them myself. Also, if you ever have ID conflicts with FTB on another pack or version of Monster, you get a file called IDConflicts.txt added to the minecraft folder. It'll tell you what ID's from what mod are conflicting with which pre-existing ID's. Just go to the 'config' folder and switch out the ones that are conflicting. They even give a suggested range of ID's to use!