Guild Wars 2 has up to now only been available on the PC, but just a few weeks after the game is hitting the Mac, albeit in its beta form.
Of course if you were extremely motivated, you could just use the Parallels Desktop software or bootcamp and load the game on a Windows partition.
In order to get the beta of the ground and on Mac PCs, ArenaNet has had to partner with TransGaming in order to use their Cider technology which basically translates content meant to run on Windows to a Mac OS X operating system.
You can try out the new client, if you have an activated Guild Wars 2 game code, after which you can download it from the account management page, you will however have to meet the following specifications:
Mac OS®X 10.7.X or later
Intel® Core™ i5 or better
4 GB Ram or better
NVIDIA® GeForce® 320M, ATI Radeon™ HD 6630M, Intel HD 3000 or better
25 GB available HDD space
Broadband Internet connection
Keyboard and mouse/equivalent
Alongside which you will also need one of the following Macs:
iMac 21.5″, 27″ (Mid-2010)
iMac 21.5″, 27″ (Mid-2011)
MacBook Pro 15″, 17″ (Mid-2010)
MacBook Pro 13″, 15″, 17″ (Early and Late-2011)
MacBook Pro 13″, 15″, 17 (Mid-2012)
Mac Mini (Mid-2011)
Source: Guild Wars 2