Well...
I will tell you how to set-up your wifi on your DS

If you have a Windows PC:
Go to Start > Run
typ in:
cmd
after that you will get a Dos Promt.
typ in the black window:
ipconfig
you will get a list with your IP configuration.
Now boot up your NDS and run a commercial game (like mariokart) and go to the
wifi menu.
choose the wifi. (the one you already have in your list)
Now you'll see some things like:
SSID
WEP KEY
blabla

Now we need the information from the PC we just entered in the DOS Prompt.
the things you will have to change on your DS are:
*Auto obtain IP: NO
*IP address (put the numbers in like you see it on the Dos prompt on the pc, in
your DS.. but change the LAST NUMBER! example:
If it says on your PC: 192.168.1.112
add +10 on the last 3 numbers of the ip: 192.168.1.122
*Gateway (put the numbers in JUST like you see it on the Dos prompt on the pc,
in your DS (mostly it's like 192.168.1.1))
*Subnetmask (SAME numbers as on your PC (mostly it is: 255.255.255.0 )
*Primary DNS (SAME numbers as the Gateway)
*leave the secundary DNS empty.
Test it, if it says connection successfull, save it.
Shut down the DS, boot up Beup..
I hope this will fix your problem.