Further complicating this issue:
I have found that if I look at flights on ual.com as a "guest" (i.e., not logged on yet with my MP number), THEN log in, the "green seats" (E+) STILL don't show up. I have to do something to start all over again in order to get the preferred seating to show.
So if you're seriously shopping (rather than just dreaming/browsing), you ought to login before doing any flight searches on ual.com.
Kathy