how do l get genius mixes on my iPod nano sixth generation?

See here: http://support.apple.com/kb/HT3843

B-rock

If you're still stuck on this, I just figured it out today! I've been having the same problem with my 6th gen nano for ages. Just untick 'Manually manage music and videos' in the summary tab when you plug your ipod into itunes. I always tick that but it worked for me when I changed it