How to sync songs to my iphone?

I bought some songs on my computer and a lot later on laptop - I managed somehow to transfer all the songs from my laptop to my computer and they're all in the library on the computer. But only the songs from my laptop will sync now? All the songs show up from the library but none of the ones from my computer will sync - there's a button thing which this 'on this iphone' on itunes where it lists only the ones that will sync - any idea how I get everything from that library to sync to my iphone?

You must convert the music for your Iphone