I don't work here, I am just a volunteer. I don't have much of the information needed to answer why video posting doesn't work for you on your MAC. Here is what I do know. Video posting was altered due to malicious code having the ability to alter the code for Network54 forums. Someone who knew how could write a fake video embed code and make it do things that shouldn't be done.
The end result is, as usual, people not trying to do the wrong thing are punished due to the actions of the few. Without knowing all the details, I am forced to assume that they cannot figure out how to make your video posting work without opening a vulnerability in the source code. It seems to be, at this time, MAC specific, as you are the only one currently complaining about it not working. It had stopped working for everyone for awhile. Then they got it back only for Internet Explorer users, then more later on....
Ripper
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