There are a couple of explanations as to why you are unable to playback an encrypted file.
1) Make sure you are viewing the file with an application that is on the MediaSeal whitelist. Depending on your operating system, there are many applications that MediaSeal supports, such as Quicktime, VLC, and Protools. Contact your MediaSeal administrator for further applications.
2) Make sure you have installed the latest version of the Decryptor software.
3) Make sure that the MediaSeal software is running.
(On OSX, you should see a MediaSeal Decryptor icon running in the Menu Bar of your computer)
(On Windows, you should see a MediaSeal Decryptor icon in your System Tray)
3) Make sure that you have inserted your iLok into the computer.
4) Contact the person who encrypted the content and ensure you have permission to view the content.