I'm seeing the same issue when I try to load your website in the selector. Unfortunately, the issue seems like it may be limited to your website. I was able to test a few secure and non secure pages from other sites without issue. You're welcome to open a ticket with Client Care to see if there is anything they can do.
Otherwise, I'd recommend just using your browser's developer tools or an add-on/extension to capture the CSS path. There are a number of add-on/extensions that make the process very easy. You would then just copy and paste the selector across to the DTM UI.