Most people can't find the 'Export to Spreadsheet' option from the Action drop-down menu due to not being on the Overview.aspx page.
But if you've modified the Overview page via SharePoint Designer, then you've probably lost the Overview menu completely
The solution is to create a New List View via SharePoint Designer and set it as Default (crucial!).
If you don't set the new view as the default page, then the Export option will still be missing.