This is a known bug and will be fixed in the release of myHack 3.2 - see this screenshot from my issue tracker for the details:
Temporary resolution:
Rename all other HFS+ partitions with OS X on them to Z_Oldname - the Z_ prefix will make them show up after the Mac OS X Install DVD in /Volumes/ so myHack will ignore them.
If that doesn't work - use traditional disk utility method to restore 10.6.3 disk image to the usb stick, then use myhack options "Install myHack Utilities", "Install Extra", & "Install Chameleon" in that order with the usb stick as your target - this will effectively achieve the same results as running myHack create installer option.