Ok let's take it one step at a time and I'll try to explain what's breaking.
The repo you added isn't an existing repo, so you won't be able to apt-get anything from it. Adding the repo using the command from the pi tutorial works for the pi because it adds a repo that corresponds to the pi's native architecture (which is armhf, and JRiver has an armhf build). The pine is arm64, for which no JRiver build exists. So when you used the "native" repo command it tried to add a .list for a repo that doesn't actually exist. The "apt-get install" command interacts with repos so that's why you're getting an error. Adding multi-arch support adds in the debian official armhf repos, which is important so that you can install JRiver's dependencies, but JRiver isn't in the main Debian repos, which is why you needed to add a repo in the first place.
So an apt-get for mediacenter21 will never work until you fix your JRiver .list to point to armhf instead of the (absent) arm64 repo. You'd do that by editing the file at /etc/apt/sources.list.d/mediacenter21.list. If you post the contents of that file on your machine, I can offer some very specific advice, but the usual way to fix it is to add the arch the repo is targeting in brackets after the "deb". So it should look something like:
deb [arch=armhf] http://dist.jriver.com/latest/mediacenter/ wheezy main
Alternatively, if you want to deal with the repo issue later and just want to get up and running, you can just directly download the latest arm build and install it directly. You can do that by typing:
wget http://files.jriver.com/mediacenter/channels/v21/latest/MediaCenter-21.0.85-armhf.deb
sudo dpkg -i MediaCenter-21.0.85-armhf.deb
That should get you a working version of JRiver (assuming you got multiarch setup correctly). You'd still need to fix the repo if you want to upgrade in a non-manual way in the future.
Hope this helps, and good luck on your journey into Linux