I'm not using the latest versions of either software but if you're talking midi, with what I've got you just click on the ".MID" icon in BIAB and save the file to wherever you want to on the mac. Then from there you can drag and drop it into Garageband.

If you're talking about real tracks you drag the instrument (or combo) into that colored bar in the upper left hand corner of the BIAB page, it'll change color after it's converted and then you just drag and drop from that bar into Garageband.

As far a where an audio file goes when it's created on the Mac, you usually get to choose where you want to put it. For example, when you create a garageband project it asks where you want to save it. The same with when you convert a garageband project to MP3.