Blood Hospital is really fun... the hordes comes in HUGE swarms that cause my computer to lag. I played with one of my friends, and we finished it on Advanced with great difficulty, because the computers are extra retarded in this campaign. There's some bugs here and there, but, overall, it's a good 3 stages. I particularly liked how they did the ending.
K2: You want to unzip the maps into the maps folder of L4D:
C

Program Files\Steam\steamapps\common\left 4 dead\left4dead\maps\ <notice the left 4 dead subfolder in the left 4 dead folder>
In order to play the map with friends, they have to have the map installed as well. Someone will also need to start a normal local server game, unless there's a dedicated server with this map. Once in the game, the person that started the local server can go to the console (~), and start the map by typing "changelevel blood_hospital_01"
EDIT: If the tilde (~) doesn't bring up the console, then you'll need to enable it. Go to Options > Keyboard/Mouse > Set Allow Developer Console to Enabled.