The DHCP settings of the access point and DS don't appear to be compatible.
What to do:
Please check the DHCP settings of your router and try again.
If you need assistance,
Click here and select your router manufacturer and model number. If available, our router pages can provide specific information on where to locate your router's DHCP settings.
What is DHCP?
DHCP is a setting, typically "enabled" by default, that lets your router automatically assign IP addresses to the devices connected to it. This option is a setting for advanced users, and normally should be set "enabled." If you prefer to have your router's DHCP set to "disable," you will need to enter a valid IP address on the Nintendo DS, through the "manual settings" option. For information on how to do this, click here.
I dont know what that is so dont ask me...