I don't have 6.2.1 but I have checked with 6.2 and 6.3 and the answer is as follows:

If you are using the live DVD when it boots up and the boot line appears at the bottom of the screen type the following (for e.g. gnome) and hit Enter (return)

knoppix desktop=gnome

Note: You have to include the word knoppix and there is no space between desktop and '=' and '=' and gnome

Having checked I can see that there are many more desktops than gnome and kde4 available. You can see all the options by reading the following file from the Knoppix DVD:

knoppix-cheatcodes.txt in the KNOPPIX directory

The boot proceeds as normal but the associated desktop appears.