First...CS5.5 is not supported under Sierra but that's not the issue here, IMO. First thing to do is patch CS5.5 to 7.5.3 and then try to open those files. I'll bet the error message changes to tell you what version of InDesign those files were saved with because what you see there is indicative of files saved in a later version of InDesign.
Thank you. Yes it did turn out to be that my files worked on the newer version of InDesign. I posted this slightly prematurely it would seem as moments after asking for help my trial version of InDesign CC (2017) installed and voila my documents opened.
On a different note, do you know if CS5.5 is due to be supported under Sierra?
CS5.5. reached end of life years ago.