I had the same problem after signing up for ITunes Match - you have to download the music to your Mac from the cloud. From your library on your Mac, click on the cloud icon next to all the songs you want to sync with your iPod. Those songs will download to your Mac and you will no longer see a cloud icon next to them. Sync your IPod and the songs will transfer.
Time consuming and tedius - hopefully Apple is working on a better solution.