Why are some of my iTunes songs unavailable when I try to add them?

[UpAllnite Software]

The songs that generate the 'Song Not Available' popup are either cloud songs or digitally protected.   

Walkout Song DJ cannot play digitally protected songs because it does not have access to the encryption keys. 
iTunes does provide a method to re-download these older protected songs in the new rights free format if the song is still available on iTunes and if the original purchase is listed under your iTunes account.  Please consult Apple Support documentation for details.

Walkout Song DJ also cannot play the iCloud items directly.

These are items that you bought (probably on another device) but have not downloaded to the device you are using. You should be able to open your iTunes app on the current device and click on the cloud icon to download those songs locally. Once the cloud items are downloaded, you can add them to Walkout Song DJ and they will play normally.

It actually doesn't make too much sense to try to set a cloud item for a walkout song.  The batter would get a little annoyed waiting for their song to start streaming  :)