MijoVoice is easily installed as any Joomla component. That should be done through your site's backend.
Once you log in, you should open the Extensions => Install/Unistall section and click on the [Browse..] button in the Upload Package File section. Navigate to the same MijoVoice file you have downloaded earlier and select it. Then click on the [Upload File & Install] button to start the installation. It will install :
1. The main MijoVoice component
2. Front-end categories & statuses modules
3. Language files
4. Database tables (#__mijovoice_categories, #__mijovoice_comments, #__mijovoice_ideas, #__mijovoice_statuses, #__mijovoice_votes)