hello Felix
a) the pd version I can build can't create a osc~ object, or b) pd won't even build depending on which sources ( svn, git, tar.bz2 ) and build instructions I use. (Currently I'm stuck with a lua issue.) n both cases I think that the build instructions I could find are incomplete or wrong and thus my result is broken.
i think you just need to add path to your ../extra folder in preferences and restart pd-extended.
best ~/.jc