Check the router,
the router has a method of security encryption, and stated in the router there will be a section for wireless security, and it will tell you what method it uses, ie WEP AES etc, and there will also be a text entry section forthe 'key' ie the password for that type of ecryption that the router uses, your PS3 will ask for the type of encryption and the Key that you set on the router.
if in doubt use a network cable, it will bypass the need for the wireless. just to test :)