Adding Files is a bit tricky. What you do is the following

At the bottom of the body page when editing an HTML item is an area titled "Relationships". On the line titled "Associated Media, choose "File" from the dropdown and click on Open Library. A new window will open.

In the new window, drag all the files you want to use in this page from the right side to the left side ("Selected") Then go to the bottom of the page and choose close. You will now see all the files you have selected. (You can't do anything with them from there, even though they seem draggable).

In the editor just to the right of the HTML icon, you will see a far cry icon, looks like a circle over a dot with waves coming out of it. Click it. A new window titled "Related Content" comes up. Choose the File Tab Choose a file you want to add from the "Item" dropdown. Then choose "Download File" From the template. Press the Preview Button and then the "Insert" button. The window will disappear and the file link will appear in the window. Continue doing these steps until each one of the files you want is in the editor

I've found that its difficult if not impossible to do structural HTML with the editor. So what I've done is simply added all the files to a blank page. Pull up the HTML window (click on the small HTML icon). Write my page in a regular text editor using structural markup and simply copy the link information over to my regular text editor. When satisfied, spell checked, etc. Copy the whole thing to the HTML window and click update.