Search google for "flatworm exit instructions".
http://www.drsfostersmith.com/produc...m?pcatid=16551
http://www.marinedepot.com/Salifert_...ADAFFE-vi.html
Not all flatworms can be controlled with flatworm exit (FE). I think only red planaria. I suggest testing first by pulling a dozen flatworms into a container with tank water and adding the recommended concentration of FE. If most are still alive after an hour then you either need a higher dosage or you have a species not susceptible to FE. I found FE didn't kill my flatworms and months later I still worry there is FE slowly leaching back out of the sand and rock. If you do have red planaria, follow the FE instructions carefully - in particular remove as many as you can before treating.
More info on flatworms and FE:
http://theaquariumwiki.com/Flatworm
Good luck. Let us know how it goes!