I don't know exactly, but it sounds like you'd better uninstall Terragen, and reinstall, then import those items again. The stuff in the tutorial are probably presets like objects or clipfiles, which you may want to use later on, so don't delete them.
Better maybe: reinstall Terragen and first work a bit in it, get to know the layout and try making some landscapes. Plenty tutorials to check out. Later import the presets.
But as said, I never imported the presets, so I don't know what's in there.
Welcome anyway. It's fantastic software, once you get over the first hurdles.