To register your handset to that base station, you must put the base station in registration mode at the same time you put the handset in registration mode. This is how you do it. At the handset you push OK when it asks you to register. If it asks you for the pin enter 0000 (factory pin) and push OK. The handset should say "registering base 1". Then at the base press the page/reg key down until it flashes (its to the immediate right of the cradle where you rest the handset for charging). That puts the base in registration mode and now its looking for the handset and vise versa. Now watch your handsets display. If it asks to to select an intercom number, then push ok and your done. If it says "invalid pin" then you need to reset your pin on your base (thats a different problem see below). If the handset times out and never registers, you need to repeat the process a couple times, you may have a bad base or it needs to be defaulted. If you need more help you can download a manual for free or get free technical help at the link below.
If you need to default your pin, you can do so but it will default the base station to factory settings. To do so remove the power from your base, hold your page/registration key down, and apply power back to the base. Count to 15, then let off the key. Turn off the power, count to five, then apply power again and follow the steps again to register the handset and you will need to reregister every handset that was previously running on it.