hello again,
Finally found a solution.
Just have to start Snort with:
snort -A fast -c /etc/snort/snort.conf -i ppp0 -b
and it works.
I am also using a "Snort Gui". For those who are intrested:
http://www.intersectalliance.com/projects/RazorBack/
it completely fulfills my needs.
pedro