It says this release (6.3) requires Yosemite (but includes SDKs for 10.9 and 10.10).
For the record, 5.x doesn't even run on Snow Leopard.
Which is a shame because OSX 10.6.8 was the most stable, zippiest performance OSX ever and still had the nice virtual desktop. Sadly I lament the old virtual desktop. I'd still be on it if I could be.