Adding your Instagram widget on your Facebook page is really easy:
Create your widget on SnapWidget and copy the code snippet
Follow the instructions in this tutorial to create a Facebook tab application: Creating a Custom Facebook Tab App
Under Step 4 of the tutorial, use the URL in the code snippet you got from SnapWidget when you created your widget. This will be a URL in this format: https://snapwidget.com/embed/123456
Continue following the rest of the steps in the tutorial.
Once you're done your widget will display in a tab on your Facebook page :)