Challenge Not yet complete... here's what's wrong: 
You haven't installed the DreamHouse app package.

I cannot install Dreamhouse.  Not only does the link in Trailhead NOT lead to installing Dreamhouse, but even using the link provided by other learners, the system says that it can't be installed because it needs APEX classes.  I don't know what that is or where to get it because I am a new user.  (That's why I'm at the very start of Trailhead, right?)  I'm disappointed that this would be happening in the first few steps of starting to learn SalesForce.  Has anyone got a solution to this installation?   Also, is ALL of Trailhead going to be like this?