Without knowing exactly what you want to do, there are a few choices to get enemies to populate a certain area. You could:

1. Manually add some enemies down in the area.
2. Place a Military Placement Custom objective module in the area you want enemies in and make the priority high.
3. You could add them same module and force enemies to spawn there by adding numbers (units) to the override lines.
4. Place a normal Military/Civilian objective module whitelisting the area with a marker and blacklisting the rest of the map.

Number two is probably the best option if you want some randomness but beware doing so won't mean they will always attack or spawn there 100% of the time.

The custom objective module gets placed exactly where you want the units to go. The higher the priority, the more emphasis the AI commander will put on the objective. The default value is "25" so you'd probably want to increase the number. Then enter the mission with debug on. If no enemies populate it quick enough to your liking, increase the number.

That, or you could also add enemies manually using this module too. The default force units lines are all set to "0." You can spawn them there by changing the unit value.