According to Nintendo...
Error Code: 20100
The Nintendo DS is unable to connect to an Access Point. What to do will depend on where you are connecting from:
Wireless home network
Hotspot
--------------------------------------------------------------------------------
Connecting from a wireless home network:
Make sure that the DS is within 30 feet of the wireless access point and facing in its general direction.
Make sure that any other interfering devices, such as microwaves and cordless phones, are turned off.
--------------------------------------------------------------------------------
Connecting from a Hotspot
Ensure the Hotspot location does not require an internet browser to setup an account, accept a use agreement, or enter a password before accessing their Wi-Fi serice. If the location requires any of these items, you will not be able to connect to the Nintendo WFC.
Make sure that the DS is within 30 feet of the wireless access point and facing in its general direction. Signal strength can vary, depending on where the access point's signal is coming from. (Please do not sit or stand in places that will interfere with the location's customers or staff.)
Orientate the Nintendo DS so that your body or other objects (such as walls, plants, furniture, and other people) are not blocking the signal. Also, make sure there is no interference from wireless phones (cell phones are okay).
If the connection continues to fail, this Hotspot may have their router set up in a manner that is not compatible with the Nintendo WFC. We recommend trying a different location.
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
My guess, is that if you're able to connect to Nintendo Wi-Fi Connection with your other games, then the server for WarioWare: D.I.Y. could be down.