According to Macintouch :

GPS

"Bob Grip
I purchased Garmin's Map Update 2008 for my StreetPilot c330 because it claimed to be compatible with "Any Intel-based Mac or PowerPC G3 or later Mac..Mac OS X 10.4 or later.."
It wouldn't work with either my dual 2 GHz G5 or MacBook Pro. Each time I tried to enter the Product Key, I got an error message that said, "We were unable to retrieve unlock codes from your GPS. Please ensure that it is plugged in and turned on. If you continue to have problems, contact Garmin Customer Support."
So, I did, and got the following response:
"Thank you for contacting Garmin International.
The update will not work on a mac. You will need to take it to a pc and perform the update."
So much for Mac compatibility!
MacInTouch Reader
I also purchased the map upgrade for my c330 and it did update as promised. One caveat, this was before I upgraded to Leopard.
The unit must be on for it to work and the Garmin web updater must be current. The process was very, very slow. In the end you will not have enough memory in the unit to store the Canadian maps, so you will be given a choice just to load the US maps.
If you have not already, try running the web updater to bring the unit's operating software current. Then try again to load the maps.
BTW, the data itself apparently does not reside on the CD, so there is a process going on via the web during the update. Looking at all the activity on my router I'd say a lot, if not all the map data is downloaded from Garmin's web site."

My brother ran into the same problem, seems that is the only model that requires a PC to run the update, luckily he has some PCs at work.