Ok this is pretty simple
Monster example.
Step 1: Find the location where you want the mobs to spawn.
Step 2: Open the map your editing with Rena's map editor.
For example map 5 Aeven.
Step 3: Select NPC spawn editor on the drop down box and click Run.
Step 4: Enter the X and Y (Where your mobs will spawn)
Step 5: Enter NPCID set peramiter to 180 and range to 3 (Allows the mob to move around at will)
Step 6: Enter the amount of monsters you want to spawn in this area under count.
Final step: Save the Spoawn Editor, then save on map editor, open your EO go to the map type $rem and walla !! You've got new mobs :)
Boom