Union Square is bordered by 16th Street, University Place, Fifth Avenue, 14th Street. Broadway also runs around it, coming in at the northwest corner and going around to the southeast corner.
Max Brenner's is just south of the southeast corner, on Broadway (approx 13th Street)
I really want to try Max Brenner's, but I never have the patience to wait the 2+ hours for a table so, if I remember to make a rezzie there one of these days, I'll let you know!!
By the way, if you sign up at Not For Tourists, you can print all their maps for free. Maybe you should get one? I find it to be an indispensible resource that fits in your pocket and looks cool.
http://www.notfortourists.com/newyork.aspx
I just bought the Los Angeles edition last year. Wow!