Where can I download these at from this site? I have done it before, but that was a while ago and now I forget how to do it.

The Download Database is where you can get mods, maps, and just about anything else, SeN does not support hacks, and therefore you cannot download them there, you'll have to go to other websites, such as BW Hacks.

Some things mod-related can be considered hacks (as they rewrite StarCraft function), so if you are looking for something like that then it is more likely to be ok.

Put quite simply, as long as you're not trying to gain an unfair advantage, it's OK. With mods, players on both ends are almost always running the same thing. (When they aren't, it's usually due to a clumsy mistake!)

If you are trying to find a way to gain an unfair advantage, then we don't want you here. When you're playing, it should be for fun, and nobody should take winning so seriously that they try to gain an unfair advantage.