If you have recently had an update install for Outlook 2011 for Mac and now you can't see your local archive folders (.pst or .olm) then it's usually because the update disconnected them.  To reconnect, please click on the File menu in Outlook (in the upper left corner) then choose Import.  Then click the right arrow button, then choose which type of file applies to you (try the .pst choice first, if that doesn't work you can repeat this process and use the other option)  and now you will see a window to search for the file you want.  Instead of looking through every file and folder, just type ".pst" (without the quotes) in the search bar and when you see the pst archive folder, click it and select Import.  Depending on the size of the folder it may take a few seconds to a few minutes.  When done it will appear on the left hand column in Outlook in the On My Computer section.