When you exchange with II, Flexchange refers to an exchange made within 59days of check-in. You don't search any differently, just put in the dates from today until 59 days from now and everything you see will be in "Flexchange".
During Flexchange the restrictions are relaxed allowing you to upgrade into a larger unit, better season or better resort. However, there are still some restrictions on some resorts. Some high-end resorts will limit who can trade into that property. Therefore, you might not be able to see everything available, but there will be many more options.