Did you do a full installation (recommended for Slack newbies)? If so, you should have networkmanager. You then need to enable it, log out, and log back in:
# chmod +x /etc/rc.d/rc.networkmanager
At which point you can click on the applet in your system tray, and should see any available wireless networks.
If you just installed Xfce and not KDE, you probably missed both wicd and networkmanager during your custom installation.