if you press the "+" button, Remote Buddy will always add your 
selection to the current playlist.

And that's also the catch. If you are currently playing a track from 
your main Music Library Playlist or any other special or smart 
playlist, it's not possible to add tracks to it.

Solution: 
Start playback from a normal playlist - or just select any album or 
track for direct playback in Remote Buddy. From then on, you can add 
tracks to that playlist.

If you should have problems with the enqueuing function nonetheless, 
please let me know which actions you are performing to expose the 
problem - and which playlist (and kind of playlist - f.ex. smart, a 
playlist in the Playlists section - or elsewhere, etc.) is active at 
that time.

I'll be happy to look into and solve any problem I get to know of or 
can reproduce.

Best regards, 
Felix Schwarz