1. Click on the "include page" button
2. This will launch the "page to include" dialogue box

3. Click on the browse button. 
4. This will launch the folder tree window.  Select the file you want to include.

5. Click the [include] button
6. This will insert a macro into your page, e.g. {{includePage:page=/path/to/mypage.html}}
7. When you save the file, the included page will appear in the text of your page.  If the included page is updated, the including page will also change.