There are many anti-virus, anti-spyware, anti-whatever software program floating around that may be trying to prevent the installation of our browser plugin. Most of them notify you if it's blocking such an installation, but some may not notify you. At this time, we are not sure of which ones do which. I'm guessing that all of them have a way to temporarily disable the installation blocking.
We have created a new page for installation that may help: Install Plugin
If you discover an anti- program was blocking the installation, please let us know.