I don't seem to understand importing models from one game to Portal 2. I took the models/props_bts/bts_turret.mdl from Portal 1, and brought it into portal 2, materials and everything, but this is all I get.Screenshot (14).png. If someone can show me to a written/video guide I would be very grateful.

You need to make sure the paths for the skins of the model are in their exact, respective folders in \materials.
Ex.:
\models\props\cake\cake.mdl
The skin for this model needs to be at:
materials\models\props\cake\cake.vtf

If you've already done that and are still having trouble, then I don't really know what to tell you. But double-check your paths!

It was the wrong paths! I can't believe it!