Multiple AU's in a course

Hello,

I'm looking to build an eLearning course that has multiple tests, in one course. The way that I'm looking to structure it, is by having a main menu, that has links to each of the individual assignable units. So what I'm trying to do is have one master assignable unit, that has several other assignable units underneath it. The problem that I'm having, is in the Title Explorer, I'm trying to put the assignable units, under the master au, but it won't let me.

Has anybody built a course, similar to what I'm trying to do?

Thanks!