Can't find the notation info I need

I'm trying very hard to find things in the handbook, but I'm just not that familiar with the terminology.
I'm trying to enter a half note with a quarter note directly above it. When I put the quarter notes in, and then try to add the half note under the first quarter note, the quarter notes go away and all I get is the half note.

Can anybody point me to the handbook notation information so I can figure this out myself?