The plugin does not appear in the list of plugins and it will not allow me to install it/ deactivate.
The plugin should appear in the list of plugins only if the 'wp-simpleviewer' folder is inside your plugins directory and the 'wp-simpleviewer.php' file is inside the 'wp-simpleviewer' folder.
I would try deleting the entire 'wp-simpleviewer' folder (either via FTP or your web hosting CPanel's File Manager).
I would then download a fresh copy of the latest version of WP-SimpleViewer (v18.104.22.168) from this web page and then install it (either manually by uploading the extracted 'wp-simpleviewer' folder to your plugins directory or via the WordPress 'Dashboard -> Plugins -> Add New -> Upload' functionality).
It just says plugin file does not exist
Please make sure that you are using the 'wp-simpleviewer.zip' file (the WordPress plugin zip file) and not the SimpleViewer-Standard or SimpleViewer-Pro zip package.
I get this:
Destination folder already exists
If you see this message whilst trying to install the plugin, then please double-check (via FTP or your web hosting CPanel's File Manager) that the 'wp-simpleviewer' folder does not already exist in your plugins directory. (WordPress seems to think that it does exist so it is certainly worth checking into further.)
SimpleViewer Support Team.