It was pretty difficult for me to figure out how to use this, mainly how to import a collection db.
Initially I thought that there'd be a feature to transfer collections over to the db that osu! is using, but this is not really the case. What I had to do was load the game's database and the one to be imported. This way, collections from both databases will be present on the list. Finally, I had to save the collection, browsing to osu!'s installation directory and overwriting the old collection.db file (being sure to make a backup, of course).
Furthermore, the "download missing beatmaps" button doesn't seem to do anything useful. I had to select the collection, select all the beatmaps, and right click to download them to the "Songs" folder in osu!'s installation directory. The only missing beatmaps were ones that had an ID of 0, probably meaning that these beatmaps couldn't be found on osu!'s website any more. If this is true, then it seems pointless to attempt to download beatmaps which have already been determined to be unavailable. The more intuitive meaning for "missing" would be any beatmap that is in the collection but is not in the "Songs" folder on the local machine.
All in all, it works but is pretty unintuitive. If it's going to remain this unintuitive, some usage instructions must be provided. For now, I hope my post ends up helping someone down the road.
Thanks for making it open source though. I'm going to look into the source later and figure out what's up with the missing beatmaps and what that download button does. Might take up redesigning some stuff so it's more in line with how I think the program should operate.
Edit: Why is this distributed only as an installer? Seems unnecessary.