I have a player model that I want to use for Nexuiz. Someone on these forums made it, so I know it is actually designed to be used on Nexuiz. Only Problem is, I don't know the first thing about actually adding it to the game so I can use it.

Can someone tell me step-by-step how to add it?

Just put the pk3 of the model in your "data" directory of Nexuiz. Then you can select it in the menu, and play with it on servers that have it installed.

To put it on a server, you first install the pk3 the same way, but need to put it on some webspace too for everyone to download. Then you put models/player/modelname.zym (or md3) in the cvar sv_curl_serverpackages so the engine knows that everyone needs the player model. Third, you point to the web directory using either sv_curl_defaulturl or a line in a curl_urls.txt (on that, see Docs/mapdownload.txt).