I recently purchased a new mac laptop, and successfully upgraded photoshop cs4 to cs6 on the machine. I still have cs4 running on my old mac desktop, and it seems from looking at your policies, that I can upgrade to cs6 on the desktop under the same purchase. Is this so? and if so, how do I go about downloading the software for the second time onto the old desktop?
Yes, you can. Simply download it again on the machine. But seriously consider simply making a backup on a USB stick so you don't have to download over and over again every tiem you need to reinstall.