Are you trying to define "ped down" in the Dictionary? That could work if setup properly, but have you tried the template defaults? Those can be found on page 28 of the Maps file. I've attached an image of what you should see.
If using one of the MW vibes, use Staff Text "ringing" / "damp".
If using one of the PED vibes, use Sibelius' standard pedal LINE markings.