Shenanigans.
I was able to watch the game just fine on my laptop via espn3 last week, and I have Charter in Missoula. I also have a watchespn app on my phone where I can watch espn3 just fine as well.
I believe the key is, that after you select "Charter" from the drop down, your username and password is your login/password that you'd use to access your @charter.net email, so:
<your username>@charter.net and your password. You do need to have your @charter.net email setup first though.
In all fairness, the first evening I tried it (a couple nights before the game), I was using the Chrome browser and it wasn't triggering the login at all, and if it did, it would just hang. I tried FireFox, and it worked there, and since then, it doesn't matter if I use Chrome or FireFox.
One thing to note though is that none of the games/replays on the other "WatchESPN" channels will work. Just the ones on ESPN3.
Hope that helps.