I bought this game on steam a couple days ago, so I install it today and enter the Cerberus key when it asks me. It hasn't asked me to enter the product key though even though steam said they would. I tried to "activate a steam product" but it says my CD key is invalid; I copied and pasted the key so I'm not entering it wrong. I want to get steam achievements but I can't. Does anyone have any idea what's wrong and how to fix this? 

NO. you have achievement inside the game. ME 1 doesn't have neither. but it is inside of  the game. to me, that's not important.

maybe leave it on the stove longer?

" I want to get steam achievements but I can't. Does anyone have any idea what's wrong and how to fix this?  "

I said 'inside of game' not steam application. read carefully. you have achievement inside game. did you never play mass effect 1, 2?  
 
I am not a guy who complained about. I just answered for him. you didn't even read here?

Oh right. Wow, can't believe I didn't see that. Thanks.