Apparently the Nintendo Channel is available on the 3DS. However, I have no idea how to access it. I tried searching for both Nintendo Channel and Kirby on the eShop (thinking it is just merged with the eShop), but turned up neither the Nintendo Channel or the Kirby Mass Attack demo.
How do I access the Nintendo Channel on my 3DS?
You're misinterpreting those links. There is no Nintendo Channel on the 3DS; you also need a Wii to play that demo.
The DS demos on the Nintendo Channel work by having the Wii broadcast the demo using the Download Play feature of a DS. That feature is also available on the 3DS, so you can play the demo on your 3DS that way, but there is (currently?) no way to play the demo without the Wii acting as the middle man.
You can get select demos from the eShop now :) (I wish Mass Attack was one of em.) Also there is now "Nintendo Zone", which will get information only when you are in certain stores with your 3DS (like Best Buy) which has trivia, previews, coupons, shows, and other stuff. There is a way to connect from home but it involves fiddling with your router and I have not tried it. There is a site that lists NZ connection points in your area. http://www.nintendo.com/3ds/nintendozone
