I've given a look to Masterplan and it's quite cool and everything, but... as far as I've seen you must be a Wizards' subscriber (read: you must pay) or you won't be able to download any tileset or other element and you'll just have a useless application. Isn't there any free stuff to download? I couldn't find any by googlewandering.
There are some tilesets here and there that can be found, but for monsters stuff you have to subscribe, or hand-enter it.
The problem I have with MasterPlan is speed and memory usage; I caught it eating a gig of memory the other day, after loading a test file (consisted of, 5 plot points, I think, and no tiles or anything). It's a neat program, but I only have three of those gig things!