How do you get DSes to link up together?

We gave our daughters DSes for Christmas. They got a whole bunch of games, as well. A few of those games are two player games. We can't figure out how to get the game systems to link up so that the games can be played as two player games. The pictochat works just fine.

We followed the instructions that came with the game system. When we try to sync the game systems to play a game, the little green light blinks like the manual tells us it should. The 'host' system says "waiting for players" and the 'join' system says it 'looking for software available for download", but they never join together.

Any ideas why we can't get the games to sync up, even though we can get the chat to work?

Any info is very much appreciated!