Hi @ogil,
This is a server issue, the zip was not uncompressed successfully. So please install it manually:
1. Use some FTP client or your hosting cPanel File Manager and remove the current uploaded folder of the addon. You'll find the /wpforo-polls/ folder in /wp-content/plugins/ directory. Just delete it, make sure all is deleted and go on.
2. Download the addon from your account, unzip it, then upload the /wpforo-polls/ folder to your /wp-content/plugins/ directory. Make sure you have not double /wpforo-polls/wpforo-polls/ directories nested inside each other, please correct this before you continue.
3. Then go to WordPress Dashboard > Plugins, and activate the wpForo Polls addon.
If it still doesn't work, please find the error.log file on the root directory and in the /wp-admin/ directory. Then attach them here, we should see the real error reports.