How should I setup the PIC's LCDREF register?
You have to read the whole datasheet section for the LCD, which is section 27 starting on page 335 and is 35 pages long.  You apparently don't understand some background about how LCDs are driven in general and this hardware module in particular, else the well written description of the LCDREF register in section 27-3 on page 339 would make sense.

Read all the data sheet section on the LCD.  Don't try to get away with reading just what you think you need.  You should always want to understand the whole hardware module before attempting to use it in any particular mode.