When I download a custom map some times custom textures aren't there or I get these red error signs. What doing I need to do. Am I forgeting to put a file in the right spot.

If a map comes with custom textures, they SHOULD include directions to place them in the proper folder for use, if they're not bspzipp'd into the map itself.

If not, perhaps they're using textures from another source game. Do you own ONLY portal? or do you have hl2 etc as well?

Interitus might be on to something with the only-owning-Portal thing.

Also, some specific examples would be helpful. In particular (at the risk of sounding elitist) are these "broken" custom maps coming from here at TWP, or elsewhere?

Well I do only own portal (It sucks I am not allowed to play TF2) so that might be it. The missing textures appear on some of the maps here at TWP and I have seen screenshots where the portals are there.

The red error signs are missing models.

It's in some way annoying that some materials are shared between the games, because mappers will easily make use of those materials, while people not owning those games will not be able to view those textures and models.

On the other hand, (I think) most people owning portal, also own HL2 and such. In that way the shared material will result in much smaller map sizes.