kinda sounds like your ppp0 interface is already in permiscous mode, hence you cant put it in permiscous mode if its already running in that manner.

Also your synthax is a little screwey. It should be something like this

/usr/local/bin/snort -d -l /var/log/snort -i ppp0 &

you left out the -l for the log directory
you left out the -i for interface
you left out the & to run it in the background.