Like NeonKnight says, you need CC3 to start with. This is the base for everything else. DD3 would help with encounter maps, as it's got dungeon symbols and drawing tools for dungeon design. The symbols in the base CC3 are for world mapping. Additional symbol packs can be bought. Symbol Set 1 gives you more for world mapping, Symbol Set 2 gives you more dungeon stuff.

Be warned, there is a learning curve with Campaign Cartographer. It's not like other drawing packages like Photoshop or Gimp. It's based on CAD (Computer Aided Design) technology. Start with the tutorials and take it slow to start with. It's not a piece of software that you can just jump in to, it takes a little bit of time to get used to. Once you start, post any questions. I'm sure someone here will get back to you. There's also a Yahoo group: CC2-l