Of course, if anybody wonders about the way the ifconfig output looks like - it was mangled in the HTML conversion.
The stuffi between < and > is invisible. It was not clear wether the interface is UP and RUNNING. So, here is, once again, the code:

root# ifconfig ppp0
ppp0: flags=8055<UP,DEBUG,POINTOPOINT,RUNNING,MULTICAST> mtu 1500
inet 62.84.136.150 --> 62.84.128.250 netmask 0xff000000
root# tcpdump -i ppp0
tcpdump: ppp0: Device not configured
root#

Any idea how to solve this?

ifconfig -a

Nothing really new, bascialy the same thing:

root# ifconfig -a
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
inet 127.0.0.1 netmask 0xff000000
en0: flags=8863<UP,BROADCAST,b6,RUNNING,SIMPLEX,MULTICAST> mtu 1500
inet 192.168.0.1 netmask 0xffffff00 broadcast 192.168.0.255
ether 00:30:65:e8:0a:06
media: autoselect (10baseT/UTP <half-duplex>) status: active
supported media: none autoselect 10baseT/UTP <half-duplex> 10baseT/UTP <full-duplex> 100baseTX <half-duplex> 100baseTX <full-duplex>
ppp0: flags=8055<UP,DEBUG,POINTOPOINT,RUNNING,MULTICAST> mtu 1500
inet 62.84.136.138 --> 62.84.128.250 netmask 0xff000000
root#

I've been able to use tcpdump on my machine, and here's how.

A quick look at the man page shows that in previous incarnations of this utility you had to be root to use it. I simply ran a sudo command, and presto, I have all the headers I can eat.
Command is as follows (a simple "-i" flag in the command lets you pick the interface):

"sudo tcpdump -i ppp0"

enter your password (if you're system admin), and you're good to go.

Hope this helps.