You have to look at the documentation of the mission and the data access etc: MMS_Data_Access_f3. We cannot tell you how to handle the data from a satellite mission (that's brilliantly interesting!) that we're not involved with. The only thing I can say is that you simply have to extract the velocity-components paralell (In swedish we have the good sense to spell it parallell, as it for visual reasons should be.) and perpendicular to the (back-ground) magnetic field:
B = read_B_field(); % you make this happen. B should be [1 x 3] for the following to work
v = read_v_cartesian(); % you make this happen. Same for v
v_par = dot(v,B)/norm(B);
v_perp = v - v_par*B/norm(B);
If you have B and v for a whole time-series the most efficient calculation might be to write out the explicit formulas for dot and norm in the above equations.
HTH
