I suspect the wep key you have is the one printed on the box, which was the original, factory set, wep key, and has probably been changed.
If you don’t know what it was changed to, and dont, have the admin password, then you need to reset the box, and log in with the default username and password admin/admin or just a username admin.
If you look on the box, you will see a small hole. push a paperclip into the hole and you will feel a small switch is pressed. Apply the power while the switch is pressed, and wait for about 5 to 10 seconds, then remove the paperclip. Wait for the box to load, give your PC Ethernet port the IP address 192.168.1.2 with a subnet mask of 255.255.255.0 and connect the PC to the router switch ports. Open your browser and connect to http://192.168.0.1 where you should then see the login page for the router.
Unless D-Link have changed things, that was always the recovery procedure (I apologise if it is not, but it has been a while since I had one of these to configure). Once you are at this stage, you can then configure any WEP key you like.
On a final note, I would advise you use something other than WEP, as it is not very secure. The router supports WPA, so I would recommend you use that instead.