I put that in the .cfg, as so:

mmodel "vegetation/tree02"
mdlalphatest 0
md3alphablend 0.5

mmodel "vegetation/tree04"
mdlalphatest 0
md3alphablend 0.5

But the game simply told me it wouldn't load them.

Also, where do I insert the third value (regarding the previous comment)?

Thanks for the help guys.

a mapmodel is an entity. in all entities, if you press 1 and scroll you change the entity's first value, with 2 and scroll the second and so on. If you edit the third value of a mapmodel, a small circle will appear around it. When the third value of the mapmodel is anything but 0, the mapmodel is noclipped.