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.
Use your best trader (your timeshare with the highest trading power) to locate the best trades using the map search function; then, call II periodically for the next several days to see if you can use one of your other timeshares to grab the exchange. In several overbuilt areas like Orlando, you can see some very high rated timeshares such as Marriott and Disney during low demand times (i.e., Jan, 2009). Best time to look is Monday mornings about 5:00 to 5:30 AM EST. Earlier then that is even better but I am not willing to get up at 3 AM. I have the sense that the II computer is uploaded at 2-3:00 AM with all the new deposits and releases that were entered late on Sunday night and the weekend transfers.