What everyone else says about your wifi woes are true, but also you have to know that the DS doesn't use some kind of browser, it just connects directly to the point, this can make the DS randomly incompatable with some access points, it's just something you have to live with.
Yeah, buy the Nintendo USB thing, it's really great (I've heard), the only downside is it's only available on the internet because Nintendo doesn't want to lose money to stores. Wasn't the third party supposed to eliminate the middleman?