I don't know anything about the configuration via the KDE control center but I use the default scripts provided with slackware.
You could edit the file /etc/rc.d/rc.wireless.conf and change the settings to reflect your configuration.
KEY = "My WEP Key"
You should also take a look at /etc/rc.d/rc.inet1.conf :
# Config information for eth1:
My network interface is eth1 and I use DHCP to obtain the IP address.
PS: I think you should consider using another method than WEP to secure your wifi connection. I takes less than one minute to break the key...
Try using WPA or WPA2.