Hehe, I guess that was a convention not really adopted by the Half-Life 1 modding community as much as it was for the Half-Life 2 modding community.

Ah... I think you are right about that.
I thought only HL2DM had the dm prefix, ah...

Well if you have downloaded custom maps before and they worked fine and you haven't messed around with Half-Life 2's internals, you can safety rule out the problem is on your end and conclude that the map is broken ;).

If you want, give me a link to the map and I can test to see if it works on my end.

Edit: Just a thought, perhaps the map is for the GoldSource engine (Half-Life 1)?

I tried it with both HL2 and HL2DM, they are in the right directory.
It seems to have no reason to not run, thats where my problem is.
dm_hogwarts was also on fpsbannana, i downloaded it from both sites and tried each, for some reason its just not working. All i really did was extract it from the zip, but i do that every time i get a zipped map, they all run just fine.
I just dont know what to do >_<

Assuming this is a Source engine map:

Half-Life 2 maps go in: Steam\steamapps\username\half-life 2\hl2\maps
Half-Life 2 Deathmatch maps go in: Steam\steamapps\username\half-life 2 deathmatch\hl2mp\maps
Counter-Strike Source maps go in: Steam\steamapps\username\counter-strike source\cstrike\maps
Half-Life 2 Episode 1 maps go in: Steam\steamapps\username\half-life 2 episode one\episodic\maps
Half-Life 2 Episode 2 maps go in: Steam\steamapps\username\half-life 2 episode two\ep2\maps

If your sticking the maps in the correct directory, than either the map file became corrupt during the download or the map file is corrupt. I've downloaded maps off a Snarkpit mirror before and they worked fine to me so I'd say the problem is specific to that map or your sticking it in the wrong directory.