Yep that is what I was looking at for weeks. I posted through out this thread what I found you need to do to get it to work.
I hope that it helps

1. ungroup all grouped objects in the model
2. with some models you will need to hide your meshes before clcking on extra parts button
3. if you are working on a already saved out RG3 rig and you made modification on it and you want to add parts to it again you need to have the name of rig selected in rig save load section.

1. There are no groups in the rig, or anywhere in the scene.
2. The skin mesh is hidden, it is not even skinned to the rig yet, or added to the rig.
3. I have not saved an RG3 file yet. The rig began as a Base Human.
It still crashes.
It’s a very simple rig, Base Human, added 1 toe, 5 fingers, 2 twists in upper and lower arms, altered proportions to fit the character mesh.
The additional rigging it a belly bone (the character is a cartoon style pregnant woman) with position and orientation constraints to 2 point helpers, one attached to the ribcage, the other to the pelvis, that’s all. I tried unlinking the points so it had no additional parts connected to the rig, but it still crashes.

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I feel your pain. Sorry I can not help you on this one. Sound like you may need to stand on head in corner for five minutes. That may help. :)

Hi everyone. I was having a problem with max locking up as soon as I clicked the CAT motion editor in x64 2011, I opened up the file in 32bit max and the problem is solved.

Just to update this thread that has re-surfaced.
I believe that this bug with Extra Parts or Add Rigging is now fixed in Max 2011.