I've installed VMWare viewer 4.6 from the URL of my work. Every time I return to that URL it keeps saying it is not installed instead of starting the viewer. It doesn't make a difference using IE or Firefox.
The problem occurs on a Sony Vaio with WIndows 7 64bit professional. I've done the same on XP and there I don't have this problem?
I can't answer your problem of the browser not recognizing what you have but as an alternative you can simply utilize the client without going into the browser. As it stands now in the 5.1 version they have stopped launching the client via browser all together.