Yes, this is a disturbingly simply question. I'm used to having DLC ping in-game with some kind of blatant e-mail message about new missions being available but so far I have heard nothing.  I am in the middle of an Insanity playthrough and haven't progressed very far into the main storyline.   
 
Does anyone know how far I need to go before the Shadow Broker missions become availalbe? 
thanks

I think you just have to get to Disc 2.

You need to get to Ilium. Then you can chat to Liara about it.

after finishing colony horizon mission, you need three more dossiers. then you go to Illium, talk with Liara. 

Thanks peeps, it's been a while since I played thru the game so totally forgot about Illium.  Looks like I have some gaming to do.

Thanks to this question. I bought the DLC like a year ago or so a few months after I finished the game actual game and then put the game away because I wanted to play a bunch of the DLC all at once and forgot then how to access various missions. Nice thread topic!