Sounds like you may have a bigger issue than load here... if you replace a bulb with led's the indicators should blink at double speed and the load's there to slow it down. This happens because the flasher sees the led's as a blown bulb due to their low internal resistance.
Generally if they don't flash at all and either stay on or fail to light the flasher's blown and needs to be replaced. To check it, disconnect the led's on one side and wire a bulb in place instead and see if they go back to behaving normally.
If you need to replace the flasher unit get one that's "non load dependant" or is designed for led's - that way you don't have to worry about fitting a load resistor at all (in fact that's a better option even if the current flasher's ok, why stuff with the wiring any more than you have to?). They're not expensive and you can get them from any half-decent auto parts shop.
Matt
