If the MOBI has DRM, you should first confirm that you can read it with (say)
Windows MobiPocket Reader. If not, redownload it from the original site for you copy of WMR. Then you can use this app's PID when striping the DRM. The easiest approach is likely the 3rd party DRM-stripping plugins for Calibre that work at the import stage. See Apprentice Alf's Blog for all things DRM related.