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I just spent a fun 5 minutes on the same quest. Maybe there's some maintencce going on. I wondered if there's some reason such as not giving credit to people who answered my questions, but I guess that's being a bit paranoid. Best try again later, I guess.
 
When you choose a product, it brings you to the main discussion page, at the top are all the subcategories and the newest posts from all the categories below that. You have to choose an appropriate subcategory, then the link to post new should appear. It is working for me, sorry if I am telling you something you already know.
 
In the few months I've been using the Apple forums, you've never been able to post in that "top-level" forum. You always had to do it in a subcategory.
 
They changed it at some point, I am thinking a while back. I remember it took me a while to figure it out.
 
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