If you add a movie via Insert, Movies And Sounds, Movie From File, you can't change the playback speed.
The file will need to be saved on your computer from Dropbox and the path of your file would be where the file is located on your hard drive such as My Documents/filename.ppt
Step 1 needs to be done within Powerpoint -- Open the control toolbox and insert a Windows Media Player
Which year of Powerpoint do you have?
How to Set Up the Add-in
To set up the add-in, follow these steps:
How to Access the Add-in
After the add-in is installed, you can access it. To do so, follow these steps:
Did you try the steps?
You do not see the Wmp.ppa file?