At the time of writing, all Focusrite audio interfaces are compatible with the 32-bit kernel of Snow Leopard (Mac OS v10.6), with 64-bit support expected on Firewire interfaces in the near future.

Visit the product section of the Focusrite web site, and you'll find 'OS Compatibility' links on all software and computer hardware pages, providing information on all compatible operating environments for the associated product.
Alternatively, you can use the dedicated OS checker here

Focusrite have developed an OS Compatibiltiy Checker, to provide up-to-date information on operating system support for their hardware drivers and audio software. This development comes in the light of Apple's Mac OS X v10.6 (Snow Leopard) and the forthcoming Microsoft Windows 7 operating systems.