Same problem here, 5.1.1 on an i4 and app no longer works
Edit: and here's an email response from the creator...
The tweak itself is not for iOS5+.
I'm sorry for that, but Apple removed the methods needed to add attachments.
The (+) button itself's not really the problem. The tweak has a check to disable itself on > iOS5 due to that problem.
The only solution's a complete reverse engineering on the MessageUI framework, which is something that would take a lot of time.
I'm sorry I cannot refund you either.
I don't think it will ever be ported to iOS5, because Apple decided to add that feature in iOS6 (they copied me! aww!) and to deliberately break my tweak by removing the methods I used.