Hi Bryan,
Is your organization configured to allow SSL only? If so, the service may be getting blocked by the browser when add item attempts to make info requests to get the service information. You can switch switch the service URL you're attempting to add to HTTPS (though it doesn't look like the server supports SSL in this case).
You should also be able to override the browser block - the behavior for each browser is different, but you might see something in the URL input box. Like a shield or something else that indicates a resource has been blocked. Clicking the shield will allow unsecured http resources to be added (be aware when doing this, it means the page is no longer fully secure. So this should only be done with trusted http resources). Here's Chrome for example:
Here's some info with links that go into more detail about each browser's mixed content support / behavior.
Troubleshoot—Help | ArcGIS
Thanks,
Chris