McDonalds can't sue Newgrounds, because nowhere on the Newgrounds front page was the word "McDonalds" even used.
As for the what looks like the McDonalds logo added to the Newgrounds logo, the tank adequately blocks out most of the big M so nobody can prove that it is actually the McDonalds logo. Tom Fulp has really thought this joke out. You Rock, Tom!
As for calling the site McNewgrounds, McDonalds can't sue people for adding the letter "Mc" to a name, otherwise they would be suing every other Scottish person (McCormick, McDougal, McMacpattywac... ect).