Adding textures, ie grass or snow

[kuram]

How is it done?  Ive been experimenting with terragen recently and all i can do is normal mountains and play around with the clouds and water, havent been able to do anything really advanced

[AndyWelder]

You should start with reading some tutorials first.
For starters here are some links to good tutorials:
The Official Terragen Frequently Asked Questions: http://www.planetside.co.uk/terragen/guide/tg_faq.html
Clouds: http://private.addcom.de/golem/tg/wolken/wolken.html (German)
Water: http://private.addcom.de/golem/tg/wasser/wasser.html (German)
Surface: http://oshyan.ashundar.co.uk/articles.html (English)
Official Surface Guide: http://www.planetside.co.uk/terragen/smgdl.shtml (English)
Carol Brooksbank's tutorial: http://www.caroluk.org.uk/terratut/ (English) Very extensiv!
JRSs Terragen™ tutorials: Zipped PDF file download (1,15 MB)
me109's Reference Guide for water setings:  Water research

I'd advice Carol Brooksbank's tutorial to start with.

[TheBlackHole]

If you want snow, use a white surface layer on top of everything else. Make it kinda bumpy and set a slope limit.