in order to use Cubase 64bit on the Mac you have to upgrade to Cubase 6 (as the Mac version of Cubase 5 is not 64bit) and run it on Mac OS 10.6 (it might theoretically work on 10.5 also but is not supported).
You don’t necessarily need to run the 64bit kernel of 10.6, most Macs boot with the 32bit kernel by default. Using the 32bit kernel doesn’t prevent the system from addressing more than 4 GB, it’s done via Physical Address Extension (PAE) and this way the 32bit kernel of Snow Leopard can address up to 32 GB of RAM. If you have a Mac Pro from 2010 which can hold up to 64 GB of RAM then you need the 64bit kernel in order to address all that memory (these Mac Pro’s boot with the 64bit kernel by default).
If you still want to run the 64bit kernel on an older machine (that is capable of running it) you either have to press “6” and “4” simultaneously every time you boot or edit the “com.apple.Boot.plist” file (as described here).
The advantage of the 32bit kernel is that you can use 32bit drivers for your hardware (in case your audio or midi interface doesn’t have 64bit drivers yet) while still running 64bit applications on top of it that can address all of your RAM (unless you have more than 32 GB installed as described above).
The 64bit kernel will actually give you no advantage unless you want to address more then 32 GB of memory. I’ve read somewhere that the performance under the 64bit kernel should be a little better in some cases but on my system I couldn’t see any difference.
At the moment though I wouldn’t recommend running Cubase in 64bit mode on the Mac as there are very few 64bit VST plugins available: Kontakt and Omnisphere are but many effect plugins are not so you would have to run all your 32bit plugins in the bit bridge which takes a lot of CPU and maybe will also cause stability problems.
Be also aware that 32bit plugins in Cubase 6 64 bit will open up with only a preview graphical user interface and in order to work with it you have to click on it a second time which opens an extra window. But the most annoying thing is (If I remember well) that you can open only one these extra windows at a time so you can’t have several plugin windows open at once.
All in all I would wait until all or most of your plugins are available in 64 bit versions and then make the jump. Until then use Cubase in 32bit mode and either activate the memory servers in Kontakt and Omnisphere or run the 64 versions of these plugins in something like Vienna Ensemble Pro.
Hope this helps!