Not able to open SharePoint Online Site in SharePoint Designer

Hi

I have created a SharePoint Online site.When I try to open this site in SharePoint Designer 2010 or SharePoint Desginer 2007 also, I am not able to open the site.

No Sever Certificate msg is displayed.

or still if I click open msg is displayed like "Path doesnot exist.Check the path and try again".

SharePoint Online site that I have created is present and I am able to edit it but not able to customize it in Designer.

Thanks