I don't think you can deal with PP files in WMM. The .pptx or .ppsx are really alien to WMM. You can, however, convert a PP file with e.g this excellent program which produces nearly any video file format - but you have to invest $49.95 (unless you want to live with the little watermark). After conversion, WMM can be used. However, I would recommend other programs because with WMM you have either an awful loss of quality or you get very, very big files. Plus, the processing takes forever. My favorite video editor is this program. (another $19.95). It has some restrictions and cannot do everything that WMM does, but if you are well organized, you get excellent results without any loss of quality and it is very fast (edits, what's called "publish" in WMM) in seconds. But this life is expensive - LOL.
PS: both programs have a trial version. So you can check them out yourself. And if you have any questions, let me know. For Machete, I plan a tutorial anyhow.