More XCode debugging details
Set breakpoints in your plug-in.
Build it into AE's plug-in directory.
Launch AE.
From XCode's debug menu, "Attach", and select AE.
Doesn't work?
Try these instructions. Oh, and buy something from Unsanity, because they're cool.
Now that your project has defined a debug executable that points down inside AE's package to the actual executable (gasp!), does XCode magically realize that it CAN attach to After Effects, even though it said it couldn't a moment ago?
It does for me.