The wireless key (password) for your network connection can be in one of a couple formats: WEP or WPA. WPA is a stronger encryption, and is not supported by the DS. WEP is supported. You should look at your router’s configuration to figure out which encryption method you are using.
Assuming that you have a WEP access point, the WFC should see it when you scan for access points and prompt you for the key. I’m just going to assume that you know this key.