Unless you need groceries to cook with, don't bother with MegaMart, it gets crowded and will take some time to get through there. There are plenty of small stores near Treehouse to pick up what you want, or you can go into Negril to one of the markets there. Your driver may be able to change enough money to get you started, or just have him stop at a Cambio.
If the timing is right, have your driver stop at Bigga's in Green Island (town before Negril) for the best jerk chicken.
There are plenty of places to eat within short walking distance of Treehouse.