Stogie

I cant join any multiplayer server as several of my files say they are unsigned (I just bought the game on steam). One example file: C\program files\steam\steamapps\arma 3\addons\anims_f.pbo What is wrong, how can this game not have "signed" files when all i have done is download it from steam? Please help, if i cant find an answer i will have to return the game. Thanks