Currently Being ModeratedJan 16, 2013 1:52 PM (in response to am2609)
Connect the the iPod to your syncing computer and update via iTunes. Make sure all the synced media like apps and music are in your iTunes library since they are not included in the backup that iTunes makes.
Backup your iTunes library just in case too.
It appears that you have iOS 4 or maybe 3 on the iPod since Messages come with iOS 5 and later. Thus, also see:
Currently Being ModeratedFeb 3, 2013 4:34 PM (in response to lllaass)
But I don't want to update. I just want iMessage.
Currently Being ModeratedFeb 3, 2013 6:26 PM (in response to am2609)
There is no way to just load iMessage.