MiwoVoice is easily installed as any WordPress plugin. That should be done through your site's backend.
Once you log in, you should open the Plugins => Add New => Upload section and click on the [Browse..] button in the Upload Package File section. Navigate to the same MiwoVoice file you have downloaded earlier and select it. Then click on the [Install Now] button to start the installation. It will install :
1. The main MiwoVoice plugin
2. Front-end categories & statuses modules
3. Language files
4. Database tables (#__miwovoice_categories, #__miwovoice_comments, #__miwovoice_ideas, #__miwovoice_statuses, #__miwovoice_votes)