This document detail how to assign footswitches to toggle effects on and off.   Click on pictures to expand them for easier viewing.

In this example, I have a wah and a distortion before the amp and a digital delay after.  I want them to toggle using the expression toe switch, FS5 and FS8 respectively.  Therefore,

-  FS5 = Screamer On/Off

-  FS8 = Delay On/Off

-  Exp Toe Switch = Weeper On/Off

1)  Choose the effects that you want to use and put them into your signal chain where you want them.  .

2)  Change to the 'controllers' screen within HD Edit and select the first effect to assign from the 'Model' dropdown (in my example, the 'Screamer').  

Note that the effects have each been assigned a default footswitch in the 'Footswitches' window but we will assume that you want to choose which effect goes on which footswitch.

3)  Choose the footswitch that you would like to assign to the effect from the 'FS Assign' dropdown (e.g FS5 for the screamer)

4)  Repeat step's two and three for all effects that you want to assign to FS1 - FS-8.  If there are any effects that you 'don't want assigned to FS1-8 then set them to 'None' within the 'FS Assign' dropdown.   The setup for our example is in the picture below - note that I have unassigned the Weeper Wah from FS4 for now.

5)  The Expression Pedal footswitch can be assigned to toggle any effect also but is typically assigned to turn the Wah on and off (note: an issue you should be aware of is detailed below).  In our example, we will assign this pedal to the weeper.  

Notes:

• You can assign multiple effects to a single footswitch - very powerful. Just use the same Footswitch each time in Steps 2 - 3.
  • The 'Footswitches' window pane will show footswitches with multiple effect assignments as 'Multi'
• You cannot control a single effect effect block using multiple footswitches.  This is a shame  (hint:  feature request ....).
• The Expression pedal footswitch toggles between EXP-1 and EXP-2.
  • If you have an effect assigned to the pedal itself (e.g. wah) then make sure that you turn the effect 'on'  when the correct expression pedal is active.
  • If you fail to do this then the effect pedal will not work. 
  • If you have this problem, try turning to the 'FX' screen in HD Edit and clicking on the button to the left of the effect's name to turn it on or off.  Now try toggling the Expression foot switch and using the effect.
• If footswitches FS-5 to FS8 change patches instead of toggle effects.  Go into system setup and change from 'ABCD' mode to 'FS5-8' mode.  Note:  in FS5-8 mode, to change patches you will now need to hit 'bank change up/down' then one of FS5 - 8 to select a patch

More details in the manual here, pages 3-7 to 3-13   ...

http://l6c.scdn.line6.net/data/l/0a06434dc7704ca4b3b26b924/application/pdf/POD%20HD500%20Advanced%20Guide%20(Rev%20B)%20-%20English.pdf