Evidently the temp graph at the top of the 2D screen uses the transducer temp sensor and to get a digital displayed temp, another temp sensor has to be connected. I'm guessing that the additional temp sensor is plug-and-play so if one is connected, the temp is displayed in the Temp. block at the bottom of the srceen without any settings required.
If you don't have Temp as a selection option in the Select Readouts menu for any position, that would confirm that a sensor is not present for that function.
I don't have that unit but the manual is available for viewing and downloading from the Humminbird web site, that is were I copied the temp explaination.