In that case, the easiest way might just be to assign the Y axis to forward and the X axis to sidways.
However, if you would like to use their holonomic drive VI, there is a nice little function for converting an x,y position (cartesian) to a direction and magnitude (polar).
I know it shows r and theta.
r stands for radius, or the magnitude of a vector.
theta is a greek character that is used for direction. It will give you direction in radians, not degrees, so you will want to make the appropriate conversion.