Here's a simple tutorial going through setting up a Spawner and enemy units.
Apologies for the visual artifacts, it seems to be an issue with my recording software.
Over the past few weeks I have put some work into our Spawner system to get an upgraded version with more features to be released as a Pro version. This has been uploaded to Unity's Asset Store and is awaiting review.
Some of the new features include:
A new custom Inspector and Editors
Multiple Spawn Locations per a Spawner
Multiple Units per a Unit Level
Coded to work with Unity 4
Lots of coding changes to try and improve performance