Getting the old "iTunes Library.itl" back
If you're getting this error from trying to downgrade, here is the fix.
First, trash the iTunes you don't want. Then install the iTunes you do want (generally the download from Apple). I think you've done this step already so on to the next.
iTunes generally makes a backup of your "iTunes Library.itl" file before it you upgrade iTunes. So navigate to ~/Music/iTunes. You'll see the "iTunes Library.itl" file there. Change its name to "iTunes Library.itl.old" to differentiate from the one you are going to put there in a second. Also, it keeps the copy in case something goes wrong.
Now go into the folder "Previous iTunes Libraries" and you should see the last itl file before the upgrade. After making a copy (just in case) change its name to "iTunes Library.itl" and drag it into the "iTunes" folder directly. Open the downgraded iTunes and it should read the file just fine.