SBS 2011: can't open web-based Sharepoint Designer

I'm quite a newbie with Sharepoint, so please bear with me if I can't even ask the question appropriately...

We installed SBS 2011 (Sharepoint 2010) a couple of years ago and it's running fine. We never used Sharepoint features.

Now we wanted to explore Workflow features so I went to http://sites and logged in as a Sharepoint administrator (successfully).

My problem is that whenever I need to go into the web-based Sharepoint Designer, I get an error. The designer is active on the server, it should be available at https://sites:987, but I always get Connection reset when I browse to that address.

I even tried momentarily bringing down the Server Firewall to check if it was a firewall problem, but the same thing happens...

A distinctive circumstance of this server is that it is not connected to the Internet (that's a company requirement, I can connect occasionally for updates, but not permanently). I've been told this shouldn't be a problem for what I'm trying to do.

Can you please advise, and help me troubleshoot? Thank you.