When the quiz Finish action is set to 'Goes to Next Slide' and the quiz is the last slide of the presentation, then the Finish button will not appear. If you want the Finish button to close the window, you will need to do the following:
1. Open your presentation.
2. Scroll to the slide that contains the quiz.
3. Click Properties.
4. Under When user passes/fails, change the selection to Closes Window.
5. Click OK.
6. Republish your file.
When the user reaches the end of the quiz, there will now be a Finish button.