Yes, it seems to be a bit confusing for many.

I have found with mine that I need to open the tray and insert paper AFTER sending the print job. Then use the console buttons on the printer to identify the paper specifics. If you make the wrong choice, remove the paper from the tray, insert it again, and then use the console to make the paper choice again (the correct one this time).

HTH - Lee (a happy 5100cn customer)

it will default to tray 1 always.  unless you specify the paper source in the printer setup. and if you are using msword.  you gotta go into the page setup, and set it mpf. otherwise it will goto the tray.  word is weird like that. it will overide printer settings.

I just got my 5100cn yesterday.  I tried using the MPF today.  I selected the MPF tray from the printer options on the computer.  Like you, I had trouble getting it to feed.  The printer kept asking me to load the paper, which I had already loaded.  I'd take it out and put it back in, then sometimes it would ask me to choose what kind of paper it was (although i had already selected the paper type on the computer).  After I selected a paper type, sometimes it would say "settings have changed", and then sometimes it would print.  Other times, it would again ask me to load the paper.  If I put in a stack of paper instead of just one piece, sometimes that would help, other times not.  If all the settings were already set from a previous print job, there was no problem.  I got so frustrated, I just loaded the special paper into the regular tray.  Before shutting the drawer, it said I had a paper jam (which I didn't).  The only way I could figure to get rid of that message was to turn the printer off.  After that, it worked fine. 

 

Next time I'll try waiting to put the paper in the MPF, as suggested.