Hrm, thats odd. Are you also building the Jitter and QC Syphon implementations from the SVN? I have only tested it with the latest SVN builds, not the public beta 1 or 2 (1 will 100% not work).
Are you also sure that Syphon.framework is properly being referenced and copied into the Screen Capture.app Content/Frameworks folder?
Yea, SVN stuff is fun, no? We ought to polish that up and do new release soon. Hrm.