You should not be using .sdf files. You should install SQL Express 2014 and connect SambaPOS to that. Here are some tutorials to help you. CE is only for single terminal setup it can also lead to slowdown and poor performance as your restaurant grows and does more transactions.
The following tutorial shows you how to setup SQL Express and the second part of the tutorial shows you how to convert from CE to SQL.
Here is a good tutorial if you have issues:
Here is the older Database configuration tutorial: