OK All -
I contacted Dell and a very nice technician talked me through the issue. I now can print shhets from the MPF.
OK I spoke with a Dell technician about this.
select menu on the 5100> tray settings> paper type> MPF> Plain1 - enter to save
Select menu > tray settings> paper size > MPF> Free Size - enter to save.
You should be able to print 81/2 x 11 sheets now. I gave up on envelopes (the printer mangles them anyway.)
Good luck all!