Please have a look here:
It lists all the formats that Maximo has as its input or output options.
I place the link there as it might change over time. If someone reads this post later on, please check, as they already announced major changes for Aug 22, thanks.
Inside of the expected zip file should be nothing but the file that is suggested in this link. FBX would be one of the file formats.
If your character in Cinema 4D is ready to be exported, and no other parts are in the scene file: Go to File> Export> FBX. If you leave all the defaults as they are, it should work. Then zip the FBX file and follow the upload instruction of the Maximo Store> Upload.
I went one time through the whole procedure, as I like to know things first hand. Here is a one minute screen capture of it: (You can download it for a better viewing experience.
Please let me know if you have any other question, I’m happy to look into it.
All the best