The default file format for Horizon Link is JPX. Your Janome 10000 will not read a JPX as you've discovered.
What you need to do is designate that you want a JEF or JEF+ format.
to do this, Don't use the "Write a Design" nor the "Send" functions.
Use the "File" function and "Save As" that way you can choose to Save it to your card and a JEF format.