well im guessing or atleast hoping you 3 who posted on here got it sorted out by now but thought id post on here to help anybody who has the same problem in the future.i had the same problem and after searching a few different forums i found that mp4's are a bit troublesome when converting them.apparently a good way to get around it is opening the mp4 in quicktimepro and then exporting it as an flv.i dont know if its that simple but i dont have quicktimepro anyway so i couldnt do it.
but here is what i did- i uploaded my video to youtube.
then using keepvid.com i downloaded it from youtube as an flv file.(you have the choice of a few different file types)
now it works in my flash perfectly.
hope it helps somebody and if anybody knows an easier way (there must be one) please let me know