It isn't very hard to do. Just use the yabbpack if the mod author offers it.. otherwise, you can use boardmod, which is pretty simple as well.
For yabbpacks, all you have to do is go into your admin area, click "yabbpack center" then add the server that the mod author has his/her mod's stored. Then go to "download new yabbpacks" click their server, download the mod you want.. then install it.
If they don't offer it in yabbpack, you could always bug them to do so.. or get boardmod.
http://www.boardmod.org/ and use it to install mods. Mods that must be installed using boardmod will have the extension *.mod.
If you need to know how to use boardmod, there should be some posts around here somewhere detailing it for you so that you can't really mess up unless you deliberately try to. If I get bored later, I might go dig one up for you.. otherwise you could use the "search" option in the menu way up above to find it.