A couple of questions. Hopefully you can answer them, and I apologize in advance if any of them seem obvious:
1) When you first go to the Black Mirror 3 webpage, you should see an Age Gate. This is to verify you're over the correct age to play the game. Are you putting in your proper birthday/year?
2) When you land on the Black Mirror 3 webpage, are you selecting the download version of the game? The default selection is PC Box. You need to select PC Download.
3) After selecting PC Download and clicking on Add to Cart, scroll to the top of the webpage and select the My Cart button. Is your cart still empty?
4) If so, what web browser are you using?
If all else fails, maybe we can set you up directly with a download link. I'll check back in a little bit for your answers. Thanks,