It looks like what you were doing was entering Brewster-388 in the surname search box. That would then take you to a surname page where "Brewster-388" is the last name, i.e. http://www.wikitree.com/genealogy/Brewster-388
What you would want for a surname page, of course, is http://www.wikitree.com/genealogy/Brewster
Alternately, you could search for an individual by entering their real name in a person search box, e.g. search for "Willian Brewster" on http://www.wikitree.com/wiki/Special:SearchPerson or the "Full Name" search field at the top of any page.
But if you do know the ID you're looking for, navigating directly with the URL bar in your browser as Michael suggests will do the trick, i.e. http://www.wikitree.com/wiki/Brewster-388