You just have to preorder it over the phone, by calling Adorama at 800-223-2500. Adorama is preselling the Gear 360 at $399.99, which is more than the ~$350 MSRP in Korea, and more than the ~$360 price at MobileFun, but Adorama’s rep said they would match the price if it turns out to be lower, and if it turns out to be higher, they will honor the $399.99 price. They do require a refundable $100 deposit, but you can cancel the preorder anytime before it ships (you should still confirm to be sure).
Although the Gear 360 is already being sold in Korea and Singapore since April 29, it has yet to be available in the US (except through resellers). Meanwhile, online retailer Adorama had been allowing pre-orders for the hot new Samsung Gear 360 camera but has since stopped, likely because of restrictions by Samsung. Nonetheless, it is actually possible to preorder the Gear 360: