EDIT: MacMiner now includes the QuarkCoin CPU miner - check it out if you'd rather use an app for mining
First, you need to download (or compile yourself) the cpu miner app:
The above link is the compiled code (minerd) from this Github: https://github.com/rallat/quarkcoin-cpuminer As well as the dependencies it requires, curl and jansson
Unzip minerd-quark-mac, then open the 'Terminal' application in the Utilities folder to run at the command line.
Drag the minerd-quark-mac folder when decompressed in to the Terminal window after typing cd then press return, e.g.: cd /Users/name/Downloads/minerd-quark
then type the following in Terminal, replacing the pool and user information with your own:
./minerd -a quark -o stratum+tcp://www.mine-pool.net:3350 -u username -p password
If you have any problems, leave a comment or get involved at the Mac mining forums: http://macminer.net/forum/