Cannot Set Password on DI524 Wireless Router

I have a Dlink DI524 wireless router connected to a mac (OSX 10.4.11). I want to limit access to my wireless router, so I tried to set up a password by doing the following:  1. I go to 192.168.0.1, click on "Home" and "Wireless"  2. I set "Security" to WEP  3. I set "WEP Encryption" to 128Bit and Key Type to ASCII.  4. I click on Key1" and provide a 13 letter password, but when I click "apply" nothing happens and the Security status reverts to "disabled".   My computer is set to PPPoE and does connect to the internet. Any ideas on how I can successfully enable a password for my router? Thanks.