First of all, find the correct forum to post your problem in.
Looks like you're talking about Source mapping, so i moved it to Source Engine Discussion.
Its good to see another Dutch mapper joining TWHL!
As for your problem: the Source SDK is a mess, especially mapping for the Orange Box games. Valve should've fixed that about now. So i don't really have a solution for your problem. Try reinstalling the SDK, then don't forget to click the Refresh SDK Content (or something similar) so that SDK is up-to-date (this is a required step).
Are you mapping for the Orange Box games or the pre Orange Box games?