How can I convert MathCAD Prime files (.mcdx) into MathCAD 15 files (.xmcd)?

I downloaded a zip file from the PTC community from the following URL.

     ·https://www.ptcusercommunity.com/docs/DOC-3708

After downloading and unzipping the file, I have a file extension of .mcdx (relations_between_distributed_load_shear_force_and_bending_moment.mcdx).  I tried to open this file in MathCAD however; the file fails to open.  How can I get this file to open?  I am using PTC MathCAD 15.  Should the extension be .xmcd?

Apperently the .mcdx files are for MathCAD Prime.  How can I convert MathCAD Prime files (.mcdx) into MathCAD 15 files (.xmcd).  I have tried simply changing the file extention but that did not work.

Regards,

Thomas Henkel