A family of Microsoft word processing software products for creating web, email, and print documents.
The Footnotes... command should be the 1st one in the 2nd segment of the Insert menu -- assuming you are in either Print Layout View, Draft View, Web Layout View or Outline View. [It is not available in Notebook Layout View or Publishing Layout View.]
Before anything else, check in a new blank document to be sure it isn't a document-specific issue [unlikely but possible]. If the command still doesn't appear, the first step is to confirm that Office 2011 is fully updated [14.2.4 is the current version]. Once the update is applied/confirmed, run Apple's Disk Utility to repair disk permissions & restart your Mac.
If it's still missing after the restart, it can be restored in either of several ways... It just depends on whether you have any menu customizations you want to retain. If none, go to View> Toolbars> Customize Toolbars & Menus. On the Toolbars & Menus page click the name of the Menu Bar then click the Reset button & OK out.
If you do have any concern for menu customizations or if that still doesn't restore the command, reply here for additional steps.