IIRC you could just drop your Key Commands in the preference folder root when moving from C4 to C5 and it showed up ready to rock next time you started Cubase? I was greated with a “file of the wrong type” moving from C5 to C6???
Don’t fear the cowbell, though! You can click the icon that looks like a folder in the Key Commands dialog, select to show all files and then select the KC file you saved in a safe place before installing C6.
Because you did, didn’t you? 
Then it works 
Aloha H,
and a major ‘Mahalo’ for that tip.
This will save many users from many headaches.
(I know it will me).
{‘-’}
In windows 7, I had to copy/paste the .xml files from users/yourname/appdata/roaming/steinberg/cubase 5/presets/key commands into the corresponding folder in cubase 6. Note that appdata is hidden so you have to set folders to “view hidden files”, and also, there is no key commands or presets folder in cubase 6 until a preset is saved, so you should save a dummy key command preset to force cubase 6 to create the proper folder for you to paste into.
If you install OS is on X:\ then
X:\Users[username]\AppData\Roaming\Steinberg\Cubase 6\Presets\KeyCommands
has a bunch of KC presets like Cubase Classic and Sonar and a few more installed with the program
Hmm. I wonder why mine didn’t have any presets? Guess cuz it’s still beta.