If you want to use that NetworkManager applet, you first have to enable the NM daemon.
You achieve that by making its rc script executable:
chmod +x /etc/rc.d/rc.networkmanager
Then clean-up the Slackware network configuration file and remove all your network configuration:
Then reboot the computer so that NetworkManager can take over from Slackware's rc.inet1 script. The red "X" will disappear and the applet will actually be useable.