If you mean a back story, like why the T's are wherever they are and what they are doing, and hence why the CT's are trying to stop them, then you should already have that!
Come up with that little bit of a story when you're coming up with the theme, don't just throw it in at the end!
I found a pretty cool site using Stumbleupon. All those warning signs posted made me think to put it here. Create your own warning signs, with symbols and your own text.
I bet you could just save the image and convert it to a texture!
Hmm. Look up the list of items, google it or something. Nightvision should be one of them. If you go to the advanced option mode in the entity, you might be able to just write in the name of the nightvision item and set the power to 1.
We have a tutorial on this in our tutorials section. In the future, just check there before making a thread. If you are still having trouble after you read a tutorial completely and you understand it, then feel free to ask for more help!
Did you google it? Just google that error text and maybe throw in a "half-life 2". As with any error, you're not the only one to have it, but you might be the only one here.
Same. This idea needs a lot of work, but it's possible.
The problem is that pre-disaster black mesa was probably not very exciting. It was probably a lot of paper work, some experimentation, and maybe a little math?
If you could come up with something to change that, a small event, then yeah. It would require new enemies and things though.
You seem to have confused the "hint" texture with the "null" texture.
If you don't have a null texture in the wad you are using, simply make a texture in wally (a blue/black checkerboard pattern or something equally identifiable) and name it "null".
I think you should post a couple more shots of your map as well, you might have done a couple other things wrong. It looks messy to me.