Posting files to the ning is not entirely intuitive, so I'll post the directions.
To post a fie (such as your presentation), go to your "My Page." Right in the middle of the page you should see something that says "Add Text." Click on that, and then find the icon that looks like a piece of paper. Click on that, and then browse to find your file on your computer and upload it. If you have another file to add, be sure to hit return before uploading the second file.
Feel free to go to my page and see the files I've posted there.