As a quick check, have you tried selecting a zip file to see if the button appears? I ask because the button is very low contrast in the current design, which may not show up well on all monitors. However, once you select a zip file, the contrast increases greatly.
Can you ensure that you have the latest version of the plugin? If you have a version older than 2.0.0, that could explain your problem.
If you are running the latest version of the plugin (currently 2.0.1), I believe that something else is causing the issue. My reasoning for saying that is that, starting with version 2.0.0, this plugin does not modify the page used to select the theme zip to install.
