That link now says July 24th:
"For the match in Kansas City, tickets go on sale to the general public the following Tuesday, July 24, at 10 a.m. CT through ussoccer.com, by calling 1-800-745-3000, at all Ticketmaster ticket centers throughout the Kansas City area (including Hen House Markets) and the LIVESTRONG Sporting Park ticket office (open Monday-Friday, 10 a.m. to 6 p.m.; Saturday, 10 a.m. to 1 p.m.)."
I wonder if there is a way to get in on a pre-sale.