Wont let me play 'Wasteland mod' - Get the message 'modified data file'

Me and a couple friends heard about a mod for Arma 2 arrowhead called wasteland. We all go to join a server they all get in fine. But when i connect i get a message saying 'modified data file' and another one saying something about a missing gun scope. I dont take notice of it connect to server and spawn as an eagle. Any help in fixing this problem would be great. Thanks

May I suggest you use the thread of said mod, although I think it is a mission. A search for "Wasteland" will reveal the threads to you, pick the one of which version you are trying to play.