Not to diss the moderator, but that was not his question.

To answer the question, since you don't have the number pad on the right hand side, you'll need to edit the Keybind on your ps3. Press start and go over to System -> Keybind. Toggle run/Walk is the 8th option or so down, and should say NUM/ next to it. Just edit the num/ to a button combo that you can press.

Incidentally, if you did manage to toggle it, that may mean there is a shortcut on the ps3 controller that you accidentally pressed. Most of these involve L1 and are NOT on the current controller set up screen. These are unmentioned shortcuts that should probably be included in the online manual.