The model of the entity should be set in the Spawn() function for that entity.
Monstermaker is just a 'helper' entity that spawns other entities. It spawns the particular type of entity based on the Monstermaker 'monstertype' key...
http://collective.valve-erc.com/inde...t=monstermaker
What 'monstertype' are you using when you spawn your custom monster? Perhaps you should create a 'generic' monster that doesn't specify the model in the Spawn() function, let Monstermaker spawn the generic monster, then set that generic monster's model based on the new key you added to Monstermaker.
botman