Please take a look into the “CachedVsts.xml” file in the preferences folder (x:Documents and Settings%username%Application DataRenoise%renoiseversion).
Open it in a texteditor and lookup the entry "value=“PS-1.dll”. The entry has set the “mIsSynth” type to “0”. Change it to “1”, restart Renoise and it should work.
We will fix this for the next release by removing he entry from the file. Seems that there exists a effect and synth plug called “PS-1.dll”.