I concur with Spotaru and others. I've used 3DC Pro for 4 or 5 years, and recently upgraded to v7.x. You get the free version downloaded, then add your license key. It should install PRO. If not, in one of the menu items, there is a "install Pro Upgrade" (at least in v6 -- with v7, it seem to do this automatically when I entered my key).
Don't panic! 3DC has been around for awhile. It's a good, stable product. Although I also have more expensive 3D apps as well, I still keep my 3DC up-to-date because it's sometimes easier.
If I had to choose: I'd rather have the ability to create something, than know something.