Inside the Photoshop CS6 folder under Applications, there is "Uninstall Photoshop CS6" at the bottom of the list.
Run that and check "Remove Preferences". That should completely remove Photoshop CS6 from your Mac.
By the way, you can get the downloadable installer for Photoshop CS5 where you do not have a disc drive to install from your DVDs.
Download CS5 products You'll just need your serial number, and if you are away from home, it is under your Adobe ID if you registered it.