Maybe your router is not using the 192.168.2.1 ip address anymore. the only way to enable the security is to access the router's page but if you want you can run the installation cd again since it'll ask you if you want to secure your network but you need reinstall everything just like you installed it out of the box.
or you can just check the ip address of your router.

here are the steps

- use a hardwired computer and access the command prompt (for windows computers, click on start, then click on run, then type cmd)
- it'll prompt you now with a black screen, on the blinking cursor, type ipconfig/all
- take note of the default gateway address that's the ip address being used by your router
- enter the default gateway's address on address bar of your browser and it shoould prompt you for the username and password, enter the username and password of your router
- it'll prompt you now with the roouter's page, click on the wireless part and set up your security

hope this'll help you

ciao!