UPDATED
I think you have to type /newent mapmodel and then enter a number. The number needs to correspond with the file
data/default_map_models.cfg
I would suggest removing those models listed in there right now and then entered the model you want in as #1 I still need to get that aspect of Sandbox working so I don't know if it will work or not yet but you can always test in Cube 2 until I'm ready.
If that doesn't work stop in the irc that I _link_ed you to in the e-mail and ask the pros

Take care.
-mike