= Wireshark Setup =

You don't need to manually add the "wireshark" group; dpkg-reconfigure does it for you. Try removing the group (and your user from the group), run dpkg-reconfigure, add your user back to the group, and then, re-login. I verified these steps in Mint 12:
{{{
$ sudo apt-get install wireshark
$ sudo dpkg-reconfigure wireshark-common 
$ sudo usermod -a -G wireshark $USER
$ newgrp wireshark
}}}