Hello. I just bought the DSiWare title Tetris Party Live and I really was looking forward to the online function. It unfortunately it doesn't work at my 3DS system, but I do have an internet connection on my 3DS. I found this information on the Nintendo site:
http://www.nintendo.com/consumer/wfc/en_na/ds-security.jsp
So I put the mac-adres of my 3ds in my router configuration, but does that really change the security of the connection between my 3ds and my router? Or do I have to do something else to be able to game online on my 3ds? Because it doesn't work yet.

I never had this problem myself, Dragon Quest Wars and Bomberman Blitz worked online fine when they were moved to my 3ds. It's probably just your router though, some of them won't allow some online functions, like gameplay but everything else is fine with them. Good Luck.

@weirdo - The info is not really clear. What it is suggesting is that you disable any type of security (assuming your router doesn't support WEP which is the only type of security the original DS/DSi software support) and use the MAC filtering as an alternative.

But MAC-filtering is a way to make the security between the 3ds and router lower, isn't it? And if not, how to change my router from WPA2 to WEP?

That page and what you're all talking about is DS game cards on a DSi.

As far as I know if you can use the eshop with the same wifi connection, then the DSiWare games should all work.

Maybe delete and redownload it? (Unless Aqueous is right)

I would try calling Nintendo, because jdarrell is right — if you can purchase the game on your DSi or 3DS (meaning you can access the internet with it already) you should be able to use any WFC functionality with it no problem. Contact info for Nintendo in your area can be found here. :3

I have already fixed the problem, I have changed the security settings of my router from WPA2 to WEP

whatever floats your boat, then. enjoy your games :3