Sharepoint Online (2013) - Cannot Open in SP Designer 2013

Hi,

I am now using SP 2013 Online and was attempting to open my new SP site.  The error says "you can't open this location using this program.  Please try a different location."

I am confused on how I should enter the URL for my new online SP.

I have tried both <a href="http://.sharepoint.com">http://<mydomain>.sharepoint.com and <mydomain>.sharepoint.com.

I can login via the web just fine but was looking for any insight to get SPD to work.

I have also copied and pasted the working web URL after I logged in successfully in to SP Designer 2013 and rec'd the same error.

Any insight to fix this would be appreciated.

EZ