Ehhh...
Install SnackBar ipk file the usual way and restart E2.
Press MENU > Setup > System > RC/Keyboard > Extra Button Setup.
Now, press either >> or << button. You will then see a number of choices...
Choose Plugins section and then go down 'till you see SnackBar, so choose it by pressing the OK button.
Now, press the green button (Save). EXIT button.
EXIT into the channel list and start a VOD film.
Press >> or << buttons and you will see the progress bar on TV screen.
Now, then: use buttons > or < around the OK button to choose how far FF or RW you want to go, then press OK. Magic!
Voila!
P.S. If there is some kind of a function already assigned to a button you want to re-map/change, then first you need to deselect/de-assign the function first, Save it and exit. Go back and now you can do as above, to assign a function the way you want it....