The DSi seems to use different WiFi code to the DS (that most homebrew was designed for), I'm yet to see anyone report that WiFi Homebrew works on the DSi. As a test try another wifi app, something like Beup.
EDIT: Make sure Win2DS is running on your computer before you start Ds2win on your DS
I use acekard 2i.
Start up DS2WIN and select auto connect.
When it asks for IP you type in your computers IP. Find it by pressing Windowsbutton + R and type CMD. A commandline(i think thats what its called) comes up and then type IPCONFIG. And you should be able to find your IP there.
Then it prompts you for password. If you didnt enter one just press start. And there we go. You should be connected