Back at main menu go to "EXTRA" and then choose the 4th option (bottom option).
Extra menu looks like this:
This is the short story section. Not all short stories are available now. Play all of those which are marked as "NEW". Each one will take only a few seconds - no choices to make here. While playing them other short stories will be marked as "NEW". Play all 23 of them. It does not matter in which order you play them.
After you played the 22nd you'll get:
Play the last one as well.
Head out to "EXTRA" menu and choose the third option.
Head out to "EXTRA" menu and choose the second option.
Head out to "EXTRA" menu and choose the first option.
And here you'll get your new plat: