Inklingo requires Adobe Reader or Acrobat (version 7.0 or later). This error usually occurs if you are using an unsupported PDF Reader such as Mac Preview. Please note that portable readers such as the Sony eReader do not have full versions of Adobe Reader or Acrobat, and usually have trouble reading protectedpdfs.
First, check that the file really is being opened with Adobe Reader or Acrobat, not with another PDF viewer that may have been installed on your computer for other documents or browsers (e.g. Firefox). They all look similar, but Adobe Reader is the one that works for Inklingo.
Please see the FAQ about Adobe Reader, below.
Also, please ensure that your Internet connection is active.