To compute steps per mm google the procedure for the formula.
Open host - connect to printer-select Config tab - select Firmware EEPROM configuration - look down list to line that says X or Y or Z -axis steps per mm.
Modify value and select OK.
Test print - recalculate if needed.
Alternatively, if EEPROM is disabled you need to use the Arduino program and modify the value in your configuration.h file (around line 66 or so)and reflash the firmware.That will make it available next time you flash firmware.
I realize it can be frustrating......BUT......sometimes google is your friend also,
Considering that these guys are doing a heck of a lot for FREE, a little legwork after their suggestions will help also.