Open the Registry Editor. You can do this by going to the Start menu (I’m still a Windows 7 stalwart, so I can’t vouch for how this might work on Windows 8), choosing “Run”, and typing “regedit” in the Open: box.
Look for the keys:
HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Common\Open Find\Microsoft SharePoint Designer\Settings\Open Site
HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Common\Open Find\Microsoft SharePoint Designer\Settings\Open Site
Delete both of the ClientGUID keys. You can do this by highlighting the key and hitting the Delete key or right clicking and choosing Delete.