My iPod won't sync new music

This is a long explanation of my problem: I have an iPod Nano 6th gen that I've had for years with a pretty large playlist. I also just upgraded my iPhone to the 5s and have made recent music purchases. I just recently purchased a new laptop and am trying to sync the new music to my iPod. I can see my iPod being recognized in my iTunes, but I can't get any of the music to play (the text is a dim gray color), but the recent music purchases from my phone will in a separate playlist. I can't find a way to combine the 2 playlists and update all the music onto my iPod. How can I combine the 2 playlists, and sync them to my iPod? I tried going to the music settings on iTunes for the iPod, and it has "sync playlist" checked (once again, gray text and I can't change options). The only box I can check is the one at the top, but when I check it a warning window pops up and I'm pretty sure it's warning me that all the content on my iPod will be wiped clean. Help 😟