Select the brush tool, right click on the canvas area (The picture you're working on) and in the menu that pops in click on the little symbol that looks like a screw/nut. That will make a list appear with all your groups of brushes.
If you got these camo brushes installed in the right folder, then they should appear in the list under "Flecktarn", "Digital", "DPM", etc. Click on the group you want to use and the list of brushes in the menu should show you the set of brushes you have chosen.