From the manual
"Entities can be created in three ways:

* by placing them in the level by WED,
* by creating them in the level at runtime through the ent_create instruction,
* by defining them through an ENTITY* definition in the script (models and sprites only)."

A link to the online manual is in the list of links at the top-left of this forum.

Additional tutorials are available in the AUM magazines (thanks George!), at the Wiki link, at the Download link, and throughout the forums.

cheers,