NOTE: Make a back up of any files that will be used just as a precaution.
First off make sure you have Java 6 downloaded. (Do a software update if you haven't done one in a while)
Go to your Minecraft app, right click and choose "Show Package Contents".
Next open the contents folder and open "Info.Plist".
Once it's open look for the line that says
Once you have found it change the 5 in 1.5 to a 6 and then save and exit.
Once done with that open a new finder window and go to Mac > System > Library > Frameworks > JavaVM.framework > Resources > Mac OS and then copy the file named "JavaApplicationStub".
Now go back to the contents folder and go into the "MacOS" folder there and paste "JavaApplicationStub".
Next search "Java Preferences" in finder.
Open Java Preferences.
In the "General" tab there should be a list of the Java versions you have. If "Java SE 6" isn't on the top of the list then drag it to the top.
Mods should now work with minecraft.
EDIT: I'm not actually sure this will still work since Minecraft has been updated a bunch but it's still worth a try. Make sure you back up files though.