Have you enabled the WEP (Wired Equivalent Privacy) encryption,this should have the option of 64 bit or 128 bit level of security.
You will possibly have to set this up if you have not done so yet.
If your Wep is set to off,you need to go through the settings and manually set this up.
As im a novice to the wireless setups i may well be corrected on my posting,but im pretty sure its correct.
Hope this helps.