I don't think that you can do a Direct To on a waypoint on a previous page (a page that is no longer in use). I think you would need to add the waypoint manually (insert the coordinates) to a position along the route ahead, then set the route leg as 0X (X being the number where you added the waypoint.

For example:

Say I am doing the Page 2 45 leg of the route. I would then insert the coordinates for the airport into waypoint 6 (or 7, 8, 9). Then I would select WY PT CHG and enter 06 (or 07, 08, 09). This would have me flying Direct To the waypoint with the coordinates I have entered.

To get the coordinates of the airport I wanted, I would open the GPS > Press the Right Large Arrow to go to the APT (airport) page > Press the PUSH CRSR button > Press the Left Small Arrow (to set the cursor on the first letter) > Then enter the airport code using the keyboard (i.e. KLAX) > Then press ENT twice to accept, then remove the highlight. Under Position, is the coordinates for the airport, so I would enter those into the INS to set that airport as the waypoint. This method can be used for getting the coordinates of Intersections (INT), NDB's, and VOR's.