original files not found after hard drive back-up

I am switching computers from an old (2009) MacBook Pro to a newer (2011?) MacBook Air. I am hoping to take all of my music from iTunes on the Pro and put it on an external drive, and then play the music on my Air through that drive. I followed the instructions here for backing up to an external drive: http://ipod.about.com/od/restoreandreformatipod/ss/Backup-Itunes-To-External-Hd_ 4.htm#step-heading, and then followed the instructions here on how to play music off the drive on my new computer: http://ipod.about.com/od/advanceditunesuse/a/Itunes-External-Hard-Drive.htm. After following these directions, all of the files in my iTunes library show up with the "original files not found" exclamation point beside them. I tried going to Preferences and re-choosing where to route the Library from, but that didn't change anything. Is there a fix for this?? Becoming quite frustrated.