From the X6 Charger thread:

The Discharge+ function on the X6 charger requires a specific resistance to achieve the desired discharge rate. The X6 itself can dissipate up to 30W maximum. To extend this, we need to use external resistors with heatsinks. Assuming that we allow the X6 to dissipate 30W, and the rest will be dissipated by the resistor bank we make, we can use the following formula to calculate the minimum resistance of our bank:

Resistance minimum = (((Vmax * Id) - P) / Id^2 ) , where Vmax is the max battery voltage, Id is the discharge current, and P is the 30W dissipated from the X6.

This results in the following results for:

2S batteries:
10 Amps: 0.54 Ohms, 54 Watts
20 Amps: 0.345 Ohms, 138 Watts
30 Amps: 0.247 Ohms, 222 Watts

1S batteries:
10 Amps: 0.12 Ohms, 12 Watts
20 Amps: 0.135 Ohms, 54 Watts
30 Amps: 0.107 Ohms, 97 Watts

For the desired discharge rate, you will need a resistor bank of the correct value, and rated for the power dissipation. For 30 Amp, 2S discharge, it looks like 4 x 1 Ohm resistors in parallel, each rated for at least 60W, should work. Be sure to attach these resistors to a large heatsink, to dissipate the heat.

Cheers.