GEM has been updated and the symbol changes were enough to break the compiled version of Syphon for GEM.
You should:
install PD Extended into /Applications
checkout the PD source code from our repository on Google Code (we will soon be migrating to github but, ignore that for now).
Ensure that GEM.pd_darwin is not red (ie, its found).
Ensure the Syphon framework xcode project is found.
Ensure the XCode scheme is 32 bit, syphon.pd_darwin
Build.
Copy files to the proper location in PD Extended.app.
Or just download the binary I compiled at:
http://code.google.com/p/syphon-implementations/downloads/detail?name=Syphon%20for%20PD.zip&can=2&q=