> How do I activate the meta box free version app I downloaded
The free version is always free and don't need activate.
I guess you mean activate to remove the notice asking to upgrade. To do this, go to My Account page, get the license key and go to your WP admin > Meta Box > License and enter it.
Note that the menu Meta Box > License is only available if you use at lest one premium plugin on your site. If you don't use any premium plugin, e.g. only the free version, then there's no way to detect that you're a premium user and thus, no way to remove the upgrade notice.