I have only used windows 8 and 10. You should be able to put your post processor file anywhere -> Open the Post Process dialog and specify the folder’s path by selecting the … box under Configuration Folder. Then select your .cps file in the Post Configuration menu. You are getting an empty list? Try using another folder for the fun of it.
I just keep my custom MPCNC posts in a folder with all my MPCNC design folders.