Which MindManager version are you using?
In version 9 and 2012 you have to create slides before you can export to PowerPoint.
In older versions, there are 2 methods to export to Powerpoint. One method exports the entire map and the other method only exports the selected topic. I guess you have to select the Powerpoint export button at the bottom of the screen for the full export (I don't have an older version anymore so I can't check it for you).