Is APP still being developed, or have you moved on to pastures greener? ;-)
Anyway, here are two problems with version 1.3 (the latest) in OS 10.9.2.
1) When I invoke Pianoteq Stage 4.5.4 (VST or AU) it says I have to authorise it. But I already have and VST and AU plugs both instantiate without further ado in Tracktion 5, for instance. Some glitch in reading authorisations in APP 1.3, it seems.
2) APP 1.3 will not accept any SKnote AU plugins. Now, SKnote has a (to me) somewhat magical system of VST > AU conversion: you just rename the basic Mac VST by ditching the .vst suffix and replacing it with .component. Believe it or not, the AU:s thus 'converted' open just fine in Garage Band and DP 7.4.
Any chance of these glitches being fixed?