You're probably using WPA encryption on your router.
It is a limitation of DS games that they can only be played online using a WEP-encrypted connection. The DSi's 1-3 connections are WEP-only for playing games online.
The Advanced connections can handle WPA encryption but the only things the DSi can do with this is use its own browser, perform updates or access the DSiWare shop.
The two solutions are - access your router's settings and change it to WEP encryption, or get the Nintendo USB wi-fi adaptor which can turn your PC into a wireless hotspot specially for DS, DSi, Wii consoles, etc.
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You're probably using WPA encryption on your router.
It is a limitation of DS games that they can only be played online using a WEP-encrypted connection. The DSi's 1-3 connections are WEP-only for playing games online.
The Advanced connections can handle WPA encryption but the only things the DSi can do with this is use its own browser, perform updates or access the DSiWare shop.
The two solutions are - access your router's settings and change it to WEP encryption, or get the Nintendo USB wi-fi adaptor which can turn your PC into a wireless hotspot specially for DS, DSi, Wii consoles, etc.