To install a map on the server, you must do the following:
1. Click install mods, then paste in the map mods id - which you can get from the steam workshop
2. Next, go onto the command line manager and click new command line. In here you will want to give it a name in the description box, then click the checkbox next to Map and in the Map box enter the maps name, e.g. Valhalla, TheCenter, TheIsland.
3. Finally save that command line, and then click on the select button to activate it.
4. The map should now be installed! :)
TheCenter and TheIsland maps come pre installed, so for those maps you can skip step 1.