MindGenius is capable of importing Mindmanager files with the .mmap extension. In some cases maps saved in Mindmanager can have a different extension depending on the version used to save the file. If your file has a .mmat or .xml extension this is likely to be the reason why MindGenius isn't able to import the file.
In order to import the file in to MindGenius, open the file in Mindmanager and re-save it using the .mmap extension.
(Note: MindGenius is able to import MindManager maps created in MindManager version 7 and upwards. MindManager 6 and earlier maps are not supported.)