Before editing a mapped domain the page needs to be redirected to the original domain.
From your dashboard go to My Sites > click Dashboard of the site you would like to edit.
Once you're in site's dashboard go to > Tools > Domain Mapping
In the domain mapping page you see a field below "Front end redirect should be:" Click the downarrow and select either "Disabled and entered domain should be used" or "Directed to original domain".
Now you should be able to edit the pages of your site. Go to Pages > All Pages > select the page you would like to edit > click Launch Squeezefunnels
When you're done go back to Tools > Domain Mapping and set back the front end redirect to your mapped domain.