Welcome to the forum Holly.
I'm afraid I don't know anything about the DSi, but some things come to mind:
1. Are you sure that the wireless point uses WEP security, and not WPA?
2. The security key will be case sensitive, so make sure that the DS isn't in Caps Lock state.
3. If the security key includes non-alphanumeric characters, is it possible that the DS is in USA keyboard mode, and therefore not putting out the characters you think it is?
I'm probably talking nonsense, and someone more competent should be along soon.