Hi timmy,
First of all, i'm not the creator of the QC plugin, credit goes to www.lalalila.org (QC plugin source) and www.mat.ucsb.edu/~c.ramakr/ (ObjCOSC.framework source).
I just used their source code and compiled a working version for PPC/INTEL.
The problem you've experiencing is a known bug (from ObjCOSC.framework).
I was working on it last week when i had an idea :
Trying it on Leopard.
There are two news, a bad and a good one.
The bad one > the QC plugin and the OSC Framework are not working on Leopard.
The good one > Quartz Composer 3.0 supports OSC natively and it is working great with P5 !
So i don't think i will try to fix the Framework bug as Leopard is gonna be unleashed soon and the bug is only present on INTEL MAC.
Anyway if you like to give a try on the OSC QC plugin, you must edit and re-type the OSCRoute "Null" String Value and save your composition again. (by editing the value, it appears to temporarily fix the problem).
Keep your P5 sketch launched when doing this and you'll see the result on the fly.
The other solution (slower) is to launch Quartz Composer by Rosetta.