FIX is just displaying what mode the nitrous setting is in (which may or may not be changeable in the Setup menu). If this is displayed on the startup screen, either you have a non-turbo box (designed for nitrous only), or a special program that can do nitrous and turbo - this requires a relay to activate the n2o solenoid, the button input is used to tell the ctrl box to add extra fuel for nitrous use, I think only some turbo boxes sold through boondocker canada (reid hedlund) have this feature.
FYI:
N2O has 3 possible modes:
ADJ=adjustable n2o press. regulator - reads N2O pressure and adjusts fuel accordingly
FIX=fixed regulator - n2o press. is not read, fixed amount of fuel will be added (only way it will work with a turbo).
NON=non regulated (will be much higher psi), n2o press is read and fuel adjusted accordingly.
Edit: I'm not sure, but maybe some/all of the earlier turbo programs still had N2O:FIX displayed on the startup screen, so you're probably fine with what you got, but there is likely a newer program available if it's that old.
