You have to enter the wireless router or access point.
This is usually done through a web browser (the default web address can usually be found on the bottom of the device) and will be something like http://192.168.0.1 at that point you will be prompted for a username and password
Enter this and you should be also to see the settings for the device including the encryption protocol used.. if your devices supports it use WPA-PSK or WPA, WEP should only be used as a last resort because its secure as a paperbag.

NOTE :- you are better off doing this hard wired to the device with a ethernet cable because as soon as you encrypt the device your wireless access may drop until you set the computers connection up using the encryption codes.