Half Life Opposing Force question

I’ve got Half Life Opposing Force installed, but I’m unable to access the Single Player option, it is greyed out. I installed it from the retail Half Life Generations pack. The multiplayer bits work, but I just can’t access the single player mode - I know it exists!

Anyone know whats gone wrong here? I’m sure its something simple…

Just want to bump this once. Some more info: I installed upgrade patches to Half Life to enable me to play it online. I don’t think I upgraded Opposing Force. I guess I will try re-installing it if I can’t get the single player mode working…