According to Nintendo's website some routers/access points won't work with wep on. Perhaps your's is one of those?
I had a similar problem and this is what worked for me. I got the error that my ds couldn't get an ip address, but this happened to me with wep on or off. But what I did was...
Just buy a u.s. ds charger. It doesn't matter what the input voltage is for the charger. As long as the output voltage to the ds is the same and they must be sense they are sending you a usb charger. Usb ports are the same voltage anywhere.
Just mod your snes, you don't need an adapter. All you have to do is remove the two little plastic tabs inside the cartridge slot. If you don't want to mod your console just get a cheap ass snes game genie and remove the tabs from it. Thats what I did.
If you already have a wired router you could just buy an access point. Thats what I did. I had a wired router and then later bought a u.s. robotics access point. Newegg has a belkin access point for only $20 with free shipping.
http://www.newegg.com/Product/Product.asp?Item=N82E16833314201...