How are you sharing your internet? Do you access the net on your DS and mac through a wireless router, or have you got internet sharing set up on your mac to share the internet with the DS via airport?
If you want to know your WEP key for your wireless network, open up 'Keychain Access' in your account, in the utilities folder. The keychain stores all of your passwords, and if you checked 'save in keychain' when you set up WEP, it will be in the keychain under your network's id. Click on it - then there will be a checkbox at the bottom of a dialogue box that says 'Show Password'. A box will appear asking if you want to always show it, or show it once, and also will ask to verify with your login password (which is also the keychain password). Authenticate, and it will show your WEP key.
