Currently, Autodesk does not provide a direct, public download link for Mechanical Desktop 2009 on its main website. Standard Autodesk policy allows subscribers to download only the current version and up to through the Autodesk Account portal or Autodesk Assistant . Because 2009 is well beyond this window, it is considered "retired".
However, if you have a valid, perpetual legacy license, you may be able to find and use it through specialized channels. Potential Download Options Internet Archive - Autodesk Mechanical Desktop autodesk mechanical desktop 2009 download link
If your company owns a valid MDT 2009 license but lost the media: Currently, Autodesk does not provide a direct, public