You need to be in the park where the ride is, and you need to go to the Kiosk (as of now). There are cast members there to help you, and some of them are very good. They offered suggestions on how to "use up" the first 3 FP+'s in order to get to the Additional FP's (At EPCOT - use Captain EO because there are FP's available for any time).
At the Kiosk, scan you band and select the button top right for Additional Fast Passes. Clicking with your fingernail works best. You will then see a screen with all of your party selected. If you want to split up, you can uncheck some people. Then, select a ride and you will see available times.
A couple of times we were faked out as the ride listed didn't have any times available, but for the most part it worked. We always checked to see what rides had the longest wait times first, because many rides listed will have short stand-by times. For example, we got Star Tours, but when we arrived it was a walk on.