After creating your resolver, you are returned to the main page of your API. Go back to Schema by selecting it in the left sidebar. Then click the Attach button next to the mutations again and select the newly-created resolver, then click Save Resolver, leaving the other options as-is.
Attach the same resolver to the two mutations and the two queries in the schema. When you’re finished, all mutations and queries should have the resolver next to them, like this.