The appointment scheduling widget is how your customers schedule their appointments so it must be installed/activated on your store.
Use the "activate" button and the app embed will activate. Then, click the "save" button shown on your Shopify theme settings (see screen shot. don't forget this step). The appointment scheduler is now live on your store. You should now go back to the app and setup your availability and appointment types if you haven't already done that.
The save action/button referenced above is on the top right of your Shopify theme.
Optional If using a Shopify Online Store 2.0 theme:
How to determine if you have a 2.0 theme?
If you have an Online Store 2.0 theme, then an " Add block" and/or "Add section" button will appear when you "customize" your theme. If these buttons don't appear, you might have an unsupported vintage theme.
If you are sure you are using a Shopify Online Store 2.0 theme and want to add the scheduling button to your product page.
Navigate to your theme settings and navigate to your default product/or page template
Click on "Add block" (product template) or "Add section" (page template) and then under "Apps" click on "Appointment scheduler"
For your product page (example shown), after the app block is added, drag it up above your the "Buy buttons" block and then click the "Save" button.
When done with 1-3, go back to the app embed section (theme settings > app embeds) and toggle off the "appointment scheduler" and then click save (reference above).
Some templates/pages will not support app blocks or app sections.