Hiya.
In a nutshell, you have to animate the model in a 3d package and/or MED. Then, using scripting, you tell the engine to run through frames X - Y. You put all the animations in one 'file'; so frame 0 - 30 might be 'walk', then 31 - 43 might be 'run', then 44 - 50 might be 'injured', etc. In the scripting, you would tell the engine to 'play 0-30 when the W key is pressed and held'.
But you have to do all your animations in a modeling/animation package. 3DGS by itself can't 'animate'...it can only do what you tell it to do; play frame X - Y when Z happens.
Hope that makes some kind of sense. Hit up the help files and search for "animation".