I suppose that with that "é", it should be pronounced PO-KAY-MON, but all the little kids (myself included) who were big fans of this when it was really a hit just went with PO-KEE-MON. I guess you could compare it to saying "me and __" instead of "__ and I"; it's technically the correct way to do it, but so many people say it the other way that we all relent at some point or another.