The formula to convert W/m² in V/m or A/m is shown in the manual page 57.
To convert to V/m you need the formula:
E [V/m] = ROOT (S [W/m²] * 377 [Ohm]) (farfield only)
And finaly to convert to A/m you need to use the formula:
H [A/m] = E [V/m] / 377 [Ohm] (farfield only)