Well to start off, I havent played SC2 yet is it peer to peer or are the games hosted by a dedicated server?
If its a dedicated server you need to find its address(example.sc2.host18.com) or IP address either would work, then simple open a command prompt (start->run->"cmd"->enter) and type:
tracert example.sc2.host18.com
or
tracert 143.122.32.88
^^ are example hostnames and IP addresses, you need to find.
If its peer to peet based then its going to be diffrent everytime so you'll need to do a few trace routes to determine the cause, but it will most likely be in the first 5 hops.
To find peer-to-peer addresses or even the deticated server address go to command promp and type in
netstat -a
this will list all open connections, start up sc2 and get yourself in a game, minimize the game and open another command promp window, once again type in
netstat -a
from this you should be able to see what connection is new and running (can also google for info like what ports sc2 uses so you can more easilt find the connection.
All of this will just give you info on where your picking up lag, better understanding of the problem will give you new aprouches to tackle it, although I'm not sure how you'll be able to improve the lag appart from bitching to iburt :/ When you get the above working post your trace route results so we can have a look