if someone could please explain to me how to do it
i cannot figure it out myself

Could you give more details as to how far you 've got?
First you need to make sure the maps you have downloaded are still pk3s and not unzipped automatically by your browser. Then put them in the nexuiz/data folder NOT the nexuiz/data/maps folder UNLESS it is just a .bsp
This could be incorrect in some way but it works for me.
You can also put maps in the dlcache but mac os x makes this a pain to do and you only really need to access the dlcache if you want to edit maps.
Hope this helps.

well for one
it automatically unzips into a folder
ad ive tryed moving that folder into the data folder but things are wierd
and ppl keep talking to me about a dlcache
idk wtf a dlcache is lol.........

Hehe...
Mac is a unix isn't it?

So, you have your home folder,, right? It's /home/username. This can be shortcutted to " ~ "
Now, inside your home, there are hidden folders too. They have a . before their names. A lot of aplications have a hidden folder, nexuiz has one too. It's path is ~/.nexuiz
There you'll find the dlcache folder. Copy all the pk3 from ~/.nexuiz/data/dlcache into your nexuiz/data folder, and you'll be able to choose those maps too.

Welllll first go to safari preference and tell it to stop opening archives on downloading them, its's in General. Then when it downloads pk3's it will still put .zip on the end but you can just edit this out.
So box.pk3 becomes box.pk3.zip
just change it back.
...
Also to recompress pk3's I use this:
http://www.macgamefiles.com/detail.php?item=18400
To make pk3's use the jedi academy option, it works for nex idk why.
...
You don't need to access dlcache to use downloaded maps in nexuiz but it is a little tidier and will stop you having duplicate files. To make ALL the hidden files on your mac visible do this in terminal:
defaults write com.apple.finder
AppleShowAllFiles TRUE
killall Finder
...
Then just do a search for .dlcache
It makes all your directories look a mess and i always feel like Ima do something fatal to my mac while i got all this stuff showing. But what you can do is create an alias for dlcache that will still be visible when you hide all the dot files again, so you should only have to do it once.
...
And to hide them again:
defaults write com.apple.finder
AppleShowAllFiles FALSE
killall Finder
...
Last of all it is possible to make pk3's using the commandline but idk how to do this and i prefer a gui anyway. Perhaps somebody could add this?

The easiest way to find the .nexuiz folder on a Mac is like this:

1. Go to the Finder
2. Click on the Go menu at the top of the screen
3. Select Go to Folder...
4. type ~/.nexuiz

Hey nice one aceofthumbs, was wondering what i was doing wrong with the go to folder thing.