MCDX is the file format of a program named PTC Mathcad Prime, which at the time is a subset of Mathcad (15,14,...) under a new UI (and to be fair - there also are some improvements). PTC has decided that there should be no way to convert a MCDX file to any older format (even Prime3 can't be converted to Prime2).
So the answer to your question is: you cannot open those files with your program. You will have to download and install Prime to look what the file is about.
