{Plugin: Spreadsheet} Will not activate

First off, I no nothing, as I barely understand the whole wordpress thing. I installed this plugin and copied the whole html code of the program and tried to post it to a page and then finding that the plugin didn’t work, I tried to backtrack. When I did that, apparently in the “installed plugins” window, the program disappeared so I couldn’t get back to it.

So I went to download it again to reinstall- but it won’t reinstall, because it tells me that it is already installed. Yet when I go to the Installed Plugin window, it is not noted as an installed program.

Where do I go to delete what is left of the shell of the program, so I can then reinstall it?