Press the little windows button at the bottom left of your screen, and type 'cmd' (without the quotes) into the search box.
Then hit enter, and a black box should appear.
Go to your wifi connection settings on your ds (they are on the pokemon menu, under mystery gift).
Scroll down your connection info, set auto obtain DNS to No, then underneath it you can change the 000.000.000.000 to the DNS (your computers internal ip).