Installing the free version of Simple Feature Requests is straightforward.
Login to your WordPress admin area.
Navigate to Plugins > Add New.
From here, you can either upload the plugin zip file, or search for "Simple Feature Requests".
Once installed, click Activate.
Installing the Pro version follow much the same process. If you upgraded from free via the WordPress admin area, the process is automated for you.
Otherwise, follow the steps above, but upload the Pro zip at step 3. This was sent you via email when you purchased it.