If you don’t mind providing access to the site via our secure contact form I would be happy to investigate this properly. It sounds like another plugin or theme script might be interfering with Microthemer. Does it only happen on the cart page? And if so, does it happen for all elements on the cart page?
Thanks for providing access. Strangely, I didn’t have any issues creating selectors using the green + symbol in Firefox. But I did see several HTTPS error when inspecting the MT interface using Chrome’s dev tools ( I also saw errors in Firefox, but they were less informative). And these errors may be preventing Microthemer from working properly.
Your site loads a Google font using http (rather than https):