Yay, another Terragen user.
First off, which version are you using? If it is the older version (0.9.43) I might be able to help with that. If, however, it is the newer version, I won't be able to offer much help.
For the older version, there are a number of good tutorials here -> http://cwied.virtualave.net/Terragen/index.html
You could also take a look here -> http://www.planetside.co.uk/terragen/resources.shtml (note, some of the links on this page no longer work, unfortunately)
Lastly, there is my own work with Terragen, though I haven't really "tutorialized" any of it yet, but it mostly involves using the SOPack plug-in and some pre-rendering work in PSP and/or Wilbur. I'd be happy to share what I have if you've got the patience to wait for me to "detail" the processes.