Ok well instead of HL2DM choose HL2...
SDK automatically sets up the different mapping modes for each game.
If you map for a mod thats exactly the same as HL2 then thats fine but if it uses a new .fgd then you have to manually set it up.
Tell me the name of the mod?
Try out mapping for HL2 in the SDK selection screen first...Tell me if that works.
Currently your mapping for a mod that does not use HL2 prefabs...
Besides why use prefabs
You should make it yourself