Kobalos

I bought and downloaded Arma 2-CO, then I installed Six Launcher and did the updates. When I try and join a game the same message appears every time which says ***/****/****/***/a10.pbo, ***/****/****/***/ah64.pbo, ***/****/****/***/air.pbo, etc. are not signed by a key accepted by server remove files or install additional keys. I have no idea what this means any help would be greatly appreciated.