Hi rbaylis,
have you ever received a reply about this issue? I had exactly the same with 1.3.9.10 and found the reason why this is happening for me: Looking in the system.log I found:
Audirvana Plus[1081]: -[iTunesFileAdder loadWindow]: failed to load window nib file 'iTunesAddFiles'.
I then opened the Audirvana package (right click on symbol --> Show Package Contents), navigated to the folder Contents -> Resources -> English.lproj and found that the iTunesAddFiles.nib file does not exist. Instead it is in the en.lproj folder. After moving it to the English.proj folder and restarting Audirvana the dialog correctly works. Hope that helps.
awicenec