Everything you need is already included in the standard Opencart package, no uploads or changes needed:

1. Go to the Facebook likebox page, https://developers.facebook.com/docs/plugins/like-box-for-pages.

2. Create your own Facebook likebox with a link to your Facebook page, changing the available parameters.

3. Copy the generated IFRAME code, and paste it on the editor (in code mode) of the HTML Content module, in the webshop admin panel.

4. Enable the module and put it anywhere on layouts of your choice.

5. That's all, no coding or extra uploads needed - the download contains instructions and pictures.

6. Enjoy!