(1) Explain your problem, don't simply post "This isn't working". What were you doing when you faced the problem? What have you tried to resolve - did you look for a solution using "Search" ? Has it happened just once or several times?
(2) It's also good to get feedback when a solution is found, return to the original post to explain how it was resolved so that more people can also use the results.
MSP to P6 never works well, its not the XML file format, its just the difference in the two applications. If its something you need to do often, I recommend looking into a Ms PRoject plugin called XER Transfer as the best option.