It has nothing to do with the 64 bit version. I use Max 9 64-bit, too.
The last version of Prim Composer that used “bitmapSizeIndex” was 1.01. It was removed in 1.02. I can't find any reference to it in 1.1 either.
I think it has to be an installation problem of some kind.
Try this test.
Restart 3ds Max and In 3ds Max, select from the top menu: Maxscript > Run Script …
This will open a window “Choose Editor File”. Move to the PrimComposer1_1/primcomposer folder and select SculptGenMax.ms, then press “Open”. It will seem like nothing happened, but see if SculptGenMax works immediately after that.
If it does, then there is definitely an installation problem. Either the files aren't where they are supposed to be or there is a duplicate SculptGenMax.ms file somewhere.
Let me know how it goes.