Unfortunately, that library makes some assumptions about i/o classes and uses a config file rather than auto detect or polymorphism to automate things.
You will need to go in an edit LiquidMenu.h to use the hd44780 header files and object class name.
i.e.: at line 52
#if I2C
#include <Wire.h>
#include <hd44780.h>
#include <hd44780ioClass/hd44780_I2Cexp.h>
#define DisplayClass hd44780_I2Cexp
--- bill