You must be in Custom Mode. While you are on a NMEA2K view press the MENU key. Select the Setup menu and cursor down to Select Readouts use the right cursor to go the Readout menu.
As an example, you want to display Engine Trim Angle.
To display Engine Trim, scroll to Engine 1 and press the check key. This will bring up a menu of Engine readouts where you can select Engine Trim and press the check key.
Press the EXIT key to return to the Select Readouts menu