I got the same error trying to install two different plugins, phpMyAdmin and BackupWordPress. I even tried uploading the unzipped files with an FTP client to my plugins folder and it wouldn’t show up in the Inactive Plugins. But for some reason, downloading the zipped file directly to the plugins directory and then unzipping it worked – it shows up as an Inactive Plugin and then clicking on Activate works fine.
Here’s a step-by-step for phpMyAdmin (I would guess it should work for other WP plugins):
Telnet/SSH to your server.
Change to the WP plugins directory. In my case:
cd /var/www/blog/wp-content/plugins
Download the plugin zip file. In this case:
wget http://wordpress.designpraxis.at/download/wp-phpmyadmin.zip
Unzip the zip file:
unzip wp-phpmyadmin.zip
If you don’t have unzip, install it via:
sudo apt-get install unzip
Then run ‘unzip wp-phpmyadmin.zip’ again.
At this point, the plugin showed up under Inactive Plugins in the WP Admin page. Click Activate next to it and it should work.
