Nuendo 5.5 headphone trouble please help

Where can I choose the source for my main headphone jacks? There is no sound coming out but my main monitors are working. When I go to hardware setup it says no source(mute) on the headphone settings.
It works when I use the MR 816 for my computer sound, iTunes,etc but it doesn’t when I open nuendo projects. Could I reset the 816 to default settings to fix this?