What you are tryingto open is a file in which "Annotation Scales" are used. Annotation Scale was first introduced by Autodesk in AutoCAD 2008/ AutoCAD LT 2008 products. Learn more on this here:
I think I have given you enough links. Now copy your file elsewhere and give a new name. Now open the file and go ahead ignoring the message. The file will open alright. Only thing is that you will not be able to edit the mechanical symbols which have been done under annotative scaling. Probably you have to erase it and re do it if you wanted to change it.
Hope my 2 cents of advise helps. Those links will definitely help you better.
With best regards
Access a broad range of knowledge to help get the most out of your products and services.