Reserving the seats in advance is limited to UA Premier (and above), US Preferred, and Star Alliance Elites. Since you aren't in these three categories, you won't be able to reserve the seat in advance.

When you check in at the airport, you should ask if there are seats available. If they are, then the agent should move you to one. Recent talk has indicated that UA has started to charge passengers to sit in E+ so I'm not sure what that will mean for your trip. But your best bet is still to ask at the airport upon checkin.