For an Adobe InDesign CS6 file to be opened in CS5 you will need to open the file in CS6 or later and save the file as an IDML file, using the Save As or Export options from the file menu. This will convert the file into a format that CS5 can open and understand. Below is a video explaining how it is done.
If you don’t have InDesign CS6 or later then maybe our InDesign down-saving service will be helpful in getting the file into the CS version you need.