If you just want to play the maps by yourself, then all you need to do is putting the maps (PK3) in your Nexuiz data folder. ~/.nexuiz/data on Unix systems (home path) and \Nexuiz\data on Windows PC's. If the file you downloaded is a ZIP file, then you either need to rename it to PK3 or extract it with a program like
Zipeg. In this case you need to extract the mappack and copy (or extract) the maps (from the "data" folder) to the data directory of your Nexuiz installation.
If you want to create a server for you and your friends to play then you will have to set up autodownload. That means writing a curlurls and setting up an FTP server for the maps (otherwise your friends would also have to download the mappack). More on that here:
http://www.forums.alientrap.local/viewtopic.php?t=18