MXP extensions are no longer supported in Extension Manager CC because this file type has some limitations compared with ZXP extensions. You can use Extension Manager CS6 to convert them to ZXP extensions then install them with Extension Manager CC.
I installed Extension manager CC and removed CS6 version (not enough space on my drive to hold 2 complete versions of the Adobe Suite).
The CS6 version does not appear in the download manager.
How do I get it?
You can download CS6 version from http://www.adobe.com/exchange/em_download/em6_download.html . To convert MXP to ZXP, only Extension Manager CS6 is required. You don't have to install other CS6 applications.