How to run a Document Package through the Community with adding the Salesforce instance.

Navigate to the button which the Community User is using and add your Salesforce instance into the button like so: &instance=na3.

For example: /apex/loop__looplus?eid={!Opportunity.Id}&sessionId={!$Api.Session_ID}&instance=na3

Note: Change the instance (na3) to match that of the Salesforce Org being utilized.