I've never heard of echinacea helping with PMT, only boosting your immune system. I know evening primrose and starflower oils are supposed to help with PMT though.
Echinacea boosts your immune system if you are feeling under the weather, if you are getting a sore throat, if someone nearby keeps sneezing (!) or if you feel you are getting a cold or the flu. Use it only for a couple of weeks, two tablets the first day and a tablet every day for the rest of the time. You may only need it for a week. I get mine at Holland and Barrett and tell everyone about them ! Not for PMT tho.