UPDATE: I duplicated my ~/Documents/Native Instruments/Traktor 3.4.0 folder and started to poke around a little.

I found that if I deleted the 'Smart' playlists that I had set up (I use them alot, have about 15 setup), quit traktor then opened Mixo it will work... It now sees all my standard playlist and I am able to import them... So it seems like this problem has something to do with the presence of smart lists in the collection.

I also tried creating a new collection folder (basically starting from scratch) I added a couple of tracks and then made a smart playlist... I quit traktor, reset Mixo to factory settings and attempted to import the new collection... this collection would sit at parse and never present me with the playlist dialogue..

Its not a scientific test, but I think the problem firmly lies with collections that have Smart playlists...

I hope this is an easy fix... I really like this software and can see myself using it alot if I can get over this hurdle!