I'm not familiar with the up-in-the-clouds gui's in mandrake anymore, but in a console, just run
xmodmap -e "pointer = 3 2 1"
to swap the buttons over on a three button mouse. if your mouse has more buttons, you'll want to add each one at the end, e.g. "3 2 1 4 5 6 7". to set this as a permanent fixture, add the entry to ~/.xsession. If you're not comfortable with this lower level approach, feel free to wait for a more user-friendly answer!