Every time I try to download software i get this error. The amount of data transferred is variable sometimes it's 0% other times it goes to 99% stops and throws this error. I'm not sure what to do, I may call nintendo directly.
You may have wireless interference, so you may want to follow the troubleshooting steps linked to on that page. Though if your internet is just fine and isn't being blocked, the DSi Shop may be just down temporarily. So just wait and try again later.
If you still can't get in later, call Nintendo at the number listed on the troubleshooting page I linked to above. You can even use e-mail if you'd rather not call.