Yes I freaking can. Assuming you've got a valid retail copy of Legacy of the void and your game is working cause you did the first thing I told you, yup.
Here's what you're going to do. First, take the physical copy of the disk you bought and put it back in the little case it came in. It's probably paper. Then you're going to look on that case and see something that says something like "Authentication Key" or maybe "Game Key" or something like that. It's gonna look something like a whole bunch of capital letters mixed in with some numbers with little hyphen dashes separating them. You let me know if you don't see that; it's like
#XXX# - XXXX - ##XXXX - #X#X - XX####
or something like that. You hold onto that game key authentication code key thing, because that is important.
Now, once you have that, then you're going to go to around about here:
http://us.battle.net/sc2/en/At the top right of the screen it says, "Your Account." That will give you a dropdown thing that says "Log in." Now, from there you're going to log in with your battle.net username and password. which will let you know the company's record for you and what games you've got. You're going to select Starcraft II, and until you upgrade, it's going to be the second expansion with Kerrigan, HotS. You'll see the option to upgrade and it's going to say like, "Redeem a Code."
Remember that game key I told you about a little earlier. Get that and type it in.
If you did it right, then congratulations, you should have Legacy of the Void. If you didn't then tell us about it here, but DO NOT post your game key code anywhere online or something like that ok.
Alright you do that and then you let me know if it worked or not here ok?