|If WEP is disabled on the router then there is no WEP key currently being used. Once you enable WEP, you will see a line to create a key. Then enter that key on your clients.|
If your router also has WPA support (it will be on the same page) and your clients support it, definitely use that instead. WEP is easy to hack, whereas WPA is virtually uncrackable with a long enough key (up to 63 characters). WPA also prevents sniffing, as each client will have its own random key which is assigned by the router, and changes every 60 seconds.
If you check Dlink's website you should find a manual for the router you have, which should show how to configure those settings.