Ugh! How do I install this plug-in?

I tried going through the same process as you, but wasn’t allowed to because I’m not in America :( Did you try installing the player directly from it’s official website? If not, try installing it from here (either on Safari or Firefox) and see how that goes. If not, you may need to restart your computer for the plug-in to work (although I wouldn’t have thought so, it may be worth a try).
Otherwise, you may want to have a read through this thread on Apple Discussions. The problem seems similar to yours, and although I haven’t looked through all the responses in detail, it seems as though the question was solved in the end. Good luck getting it to work, let us know how you go.

I couldn’t get it to install even from the official website (thanks for the link) or restarting the whole computer, etc—and in fact in the blog you linked and others, saw that others are having this problem. Fortunately, I found what I wanted to watch on http://www.watchtvsitcoms.com/ !

Ah, bummer, sorry it didn’t help, but glad to hear you managed to watch what you wanted in the end :)