download the map

unpack the file as it is either a zip or rar file.
Extracting
If you download a ".zip or a ,rar" file you will need to extract it first before you can install. i would recommend installing winrar from www.rarlabs.com. Once installed the rar and zip files will be a small stack of books for easier identification.

.VPK Installation
Most maps and campaigns will come as a VPK file. VPK files are Valves version of packed add-ons and can usually be installed by simply running or double clicking on the file. If it doesn't run and you get a dialog saying "Windows can't open this file" you have 2 options.

1: "Select a program from a list of installed programs" and hit "Ok". This will bring up a new window, click "Browse" and navigate to "Steam\steamapps\common\alien swarm\bin" and select "addoninstaller.exe". If you checked "Always use the selected program to open this type of file" option then from now on all VPK files will now be automatically installed when you double click them.

2: is to copy each file to "Steam\steamapps\common\alien swarm\swarm\addons" and your map/campaign will be installed. Downside to this option is you will have to do it for each VPK file you download.

After you have your file installed you can view and manage it in-game via the Options->Add-ons menu.

a series of folders and text files are in the download?
basicly this is what the layout is inside a vpk file just the author is not that bright to undertsand how to make it a vpk file.
simply goto your alien swarm bin folder, and create a shortcut to the vpk.exe on ure desktop, now drag the map folder from download(normally be the maps name wioth all other files inside) onto the vpk.exe programme and bobs your uncle u have a vpk on ure desktop just follow the above vpk instructions job done.

now if u have a bsp file not a vpk or even a seires of folders and txt files then follow the step underneath.
.BSP Installation
Copy the file to "Steam\SteamApps\common\alien swarm\swarm\maps" folder. In order to play these maps you need to check the "Enable Developer Console" in keyboard options within the game. After you have that enabled you can start a game, go to the lobby and invite friends. When ready you open the console (default key is ` or ~) and type "changelevel MAPNAME" where MAPNAME is the name of the map you installed.

more maps added to the media section/downloads here, will be adding more each few days when we find them.