You must activate the Room Bounding type property on the linked model itself to be able to place spaces in a linked model. 
- If you do not already have a linked model, use the instructions in 
  Link One Model to Another to link models. 
- In a plan view of the host model, select the linked model. 
  The status bar displays the following: 
   RVT Links: Linked Revit Model: <model name>. 
   
- In the Properties palette, click 
   (Type Properties). (Type Properties).
- In the Type Properties dialog, select Room Bounding (it is deselected by default). 
- Click OK. 
Note: You cannot use IFC-based curtain elements to define rooms. If needed, add room separation lines along their boundaries. See 
  
Add a Room Separation Line.