Probably your session is not being saved on logout. Open KDE Control Center > KDE Components > Session Manager and check On Login - Restore previous session
. Then run kmix, log out and login again, it should be present...
Otherwise, you could i.e. add kmix.sh file to ~/.kde/Autostart.
Open your favourite editor, enter next code and save it as kmix.sh in your home folder:
Then make it executable and move it like this:
$chmod +x kmix.sh
$mv kmix.sh ~/.kde/Autostart/
That should solve your problem.
About networking icon... In Slackware there is knemo - network monitor. If it is not available in VL repo, you could request it or maybe use Slackware package. Otherwise, you could use wicd to manage and monitor your network connections.