Here is the box you see when you post a message:
Click on the "Add a Google Doc" button:
From here you can link to an existing Google Doc, or create a new document, spreadsheet or presentation (these options are at the bottom of the drop down). Or you can click on "Upload a File" and switch to upload a Google Docs compatible file from your computer.
Clicking the "Link a File" link will switch you back to the list of Google Docs.
Before you can do any of this, you need to setup your connection to Google. From the messages page, look under the "Import" menu.
Click the "Setup Google Connection" button to allow Heap to access your Google Docs.