If you cannot open an Acrobat version of your lesson, this may be due to one of a number of reasons:
The lesson file may have been corrupted during download -- Go back to your Download Page and try downloading your lesson again. You can also try saving the lesson to your hard drive and opening it from there.
Your version of Acrobat Reader may be too old -- This is indicated if you receive a message such as "C% Font Error" or similar. If you do not have Acrobat Reader 5.0 or above installed, you will need to go to http://www.adobe.com/products/acrobat/readstep.html to install the latest version of Acrobat Reader.