Why won't SMC won't load it into my Sansa?

I have a sansa e260 and have this mp4 file that i want to put into it.  It’s an MPEG4 (MP4) file!! i clicked Add Media and added it into the list in Sansa Media Converter and clicked convert. But it failed!! It says its an unsupported filename!! but MP4 IS a supported filetype , it even says! why won’t it work!!! help plz! very much appreciated! >:(

Did you download the latest Sansa Media Converter? SMC 236. Also remember to uninstall the old version before installing this new version. Install Quick Time player for the codec. Hope this help and good luck.