You can select the track and go to the 'Inspector' (F4)
In there you should be able to find 'File Tempo' - If that says 'Not set' then do put in the tempo there at which it was recorded
Now, still in the Inspector, one of the top ones in there should say 'Tempo' with a dropdown menu to it's right. Set that to 'Tomestretch'
Your track should now follow when you change the tempo of your song