From Valve Developer Community
A map pack is a pack of maps (.bsp files).
To "install" a map pack, do the following:
- Note which game the map pack is for.
- Figure out which map appears to be the first one in the pack. (Usually it's the one with the lowest number.) Write this name down on paper for later.
- Copy the map (.bsp) files directly into the maps\ folder of the associated game. (<Installation path>\Steam\SteamApps\common\<game>\<game abbreviation>\maps\, for example C:\Program Files (x86)\Steam\SteamApps\common\Half-Life 2\hl2\maps\.)
- If any node graph (.ain) files are present, copy them into the graphs\ subfolder of that maps\ folder.
- Start the associated game.
- Open the developer console.
- Type map <name of the first map> in the console to load the map.
- Modification - Something more than a map pack.