I don't have steam but can't you just -wadinclude the wad? do you know where the wad is becuase if the problem is that you can't use it from that location then you can copy/extract it. I know that quake II wads were inside
the pak itself maybe it's also true in that case. Any way the wad file is called opfor.wad maybe this will help you find it. : Any way if you need a pakexplorer wally is great for you can also create wads with it (there is also a program named pakexplorer that can also help you). Here is a site where you can down load it
http://www.vlatitude.com/