Are they not blinking at all, or are they flickering really fast?
If they are flickering really fast, get a new flasher relay.
If they aren't lighting up at all, they are wired wrong.
LED blinkers have to be wired correctly, if you wire them backwards, nothing will happen. If you wired them correctly, they should light up and flicker really fast. If this happens, you can either wire in the resistors, like you said, or replace the flasher unit with one made for LEDs, like the one half-way down the page, here.
Just want to make sure you don't spend more money than you need to.