No REALLY... where is it?
I'll accept any answer from a Sony employee, since no one on phone support seems to know anything about it, even though it's the only way to play many PSP games online and WAS TALKED UP ON PLAYSTATION'S OFFICIAL BLOG.
Any answer will do. Even if it's "we removed it, because we hate all PSP owners for not buying the Vita."
And really, I do swear to god that it wasn't there several weeks ago.
I had found it on messageboards around the internet of how to get to it just as you described, but it simply wasn't there. I even searched through every PSP game add-on 1 by 1.
Even called Sony's support... 2 different support people looked through the store on the phone, and they couldn't find it either. They told me to come post on this messageboard to ask where it went.
Then today, as I was going into the store just to apply my Ni No Kuni pre-order bonus, I saw there was an update to the store app. So while I was there, I looked for AdHocParty again... and it was there this time! Wohoo!