1. Did you build the addon as per the instructions in “Read Me – Building the OF addon.rtf” in the OpenFrameworks folder you got from SVN?
2. Make sure you’re linking the framework in the Xcode target for your project. If you select the target in Xcode and expand it, there should be a “Link binary with Frameworks” build phase (or similarly named, I only have Xcode 4 to hand) – if Syphon.framework isn’t listed there, drag it from inside the ofxSyphon group in the Xcode project.