... seems we'll just have to wait
When you see New! Priority Inbox in the top right corner of your inbox you'll be able to try it out.
If there is another way, I'll post directions/link here.
Supposedly 'is in beta and should be rolling out over the next week' so shouldn't be too long?
update 2010.09.03: It came today, although I did see it on a couple other accounts sooner.
I already have a number of filters for categorizing some of my incoming mail into shopping, technews, and junk labels, plus some Quick Links for filtering some of this out from my view when I want to focus on more important stuff.
However, with the filters/quick links, I have to manually add anything new..
Priority Inbox promises to do this automatically through machine learning and user training.