The museum is huge and full of treasures: the portion shown on this map represents maybe one twelfth of the galleries. Museums in the U.S. don't require payment to enter, but it is customary to pay the recommended donation ($10 is the MET's suggested price). It sounds like Holden and company made a quick visit without paying.
More notes on the map:
The entrance faces Fifth Avenue. It has a lot of stone stairs leading up to it.
John Rager wrote me, remembering the layout from the 1960's:
"The entire Egyptian section of the museum has been substantially redone since the time of the novel The narrow passageway (to the mummies) described by Salinger no longer exists. I believe that the cloakrooms are in the same place that they were before the renovation, but I'm not even sure of that. They do not look the same. the floor layout of the [Egyptian] exhibit is very different now, and the Temple of Dendur courtyard (in the glass) was not there (The Temple of Dendur was a gift from Egypt for help in saving the artifacts that are now under the lake behind the Aswan High dam. The major renovations of the Sackler wing took place in the 80s, if I remember correctly. There was a fair amount of building in the 80s and 90s.) The great entrance hall has been renovated, but not changed much - the store which is off the lobby was not there before, for example. the cloakrooms were redone but I think they are in the same place."
MET's Web site: http://www.metmuseum.org