Richard - VE7CVS
You may also get 'command not found' for tools that are in the /sbin or /usr/sbin directories, these are not typically in the path for 'ordinary' users.
If tcpdump is installed and you still get this error, try:
sudo /usr/sbin/tcpdump ...
(with ... meaning the options that you want to pass to tcpdump).
- Richard, VE7CVS