Which Mac do you have?
The first thing I would do would be to hook the computer up to the internet via an ethernet cable and install any updates.
If that doesn't work check in, System > Administration > Hardware Drivers, to see if there are any proprietary drivers available to install.
If that doesn't work you will need to identify which wireless card you have in your Mac, so that you can install the correct drivers for it.
Either run "lspci" (thats a lower case L) from Ubuntus terminal (Applications > Accessories > Terminal), or in OSX go to Apple > About This Mac > More info > Airport, this should give you some information on the card.
Once you have more information then you can check to see if it's supported, and whether there are any drivers available.