I don’t know what’s going on, but I tried to install a plugin for the first time today and had the same problem.
God news. I just found the solution! Apparently reinstalling WordPress is all it took..I think some versions of the one-click installation are flawed..
I never did install wordpress… I just use the blog as hosted by wordpress.com. Can I still use plugins?
Okay… found the answer to my own question. http://en.support.wordpress.com/plugins/