In HD500 Edit (which I presume you are using since you talk about dragging) there is a tab called Controllers. This is where you make the footswitch assignments. IN the top left field, labeled Model, select the desired FX. Then in the filed immediately below, labeled FS Assign, assign the FX to the desired FS. The assignment of all FS is visible in the FOOTSWITCHES pane, below the CONTROLLERS pane.

Don't forget to use the SEND SELECTED command in HD500 Edit if you want to load/save the changes to the HD500 device. Otherwise your changes will be lost as soon as you close the HD500 Edit program.

an additional note to jimsreynolds in the fx edit screen even though they wont be in order vertically as they are in the picture window when you scroll over a particular effect or blank box (no effect yet chosen), the proper vertical fx box or block will light up so you can see which one it is and the edit parameters for that effect.