Maybe we need a few more details?
First, have I understood this correctly?.. you have an audio track containing a loop, and, already, it doesn’t match Cubase’s metronome? You want to make the metronome match the loop (or the other way round?), and, once the two are in sync, you wish to add some loops from the Loop Browser?
What is the source of your original loop? Is its tempo (supposedly) constant? Is it just a couple of bars long, or plays linearly for the duration of the song? (i.e; without needing to be repeated).
I’d be inclined to use the TimeWarp Tool on the original audio track (plenty of details how to do that in the manual, and also many posts in these forums… some of them might still be in the Cubase 7 forum). Seeing as how the source material is a loop, you should be able to to use the TimeWarp Tool to drag a Cubase barline to match the end of the first bar (as listened to). This should then show the tempo for that bar, so then make the tempo track to that tempo for the song’s entire length. If it isn’t quite right, you’ll notice this more and more as the song progresses. So Go towards the end of the song, and see how well it is holding. If necessary, fine adjust the tempo, until it feels right at the end of the song also.
It should then be straightforward to drag your other loops into the Project.