Remove listed files

[Tidycpc 0]

Hi

I have downloaded the Arma 2 free and bought OA.

Did the necessary installation step and as soon as i try to join a server i get the following message:

It pops up with list of files and directories and it says that the files aren't signed with a key the server accepts. It tells me to remove the listed files or install additional accepted keys

Please can anybody help. Really want to get into this game.

Thanks