I think you will find that the configuration of the source search path is *very* complex, indeed too complex for a beginner. It's great for hackers with complex requirements, but there's too much to be read if you just want to make one or two simple systems.
FTR, even i was mildly irritated by this when i first approached the new ASDF config infrastructure.
but don't get me wrong, i have no ideas how to simplify it, and the docs are probably also much better these days...