I have the same problem. Turned off Windows Firewall and my virus scanner (AVG Free). I am running Windows 7 Home Premium (Dutch version, regional settings at Dutch or US didn't make a difference).
I have found a workaround: When I run DesignSpark PCB under elevated permissions (Right-click on DesignSpark and click Run as Administrator), the ModelSource window is able to connect and works fine.
But this is not a great solution of the problem, I want to be able to run the software with normal user rights.
I had the same problem but seem to have solved it. The problem appears to be in the database search engine at RS. If I put in an RS part number or a complete maunfacturer's part number I do not get the communication error problem and I can download the part into my database.
Also, using the drop down menus above the window where I type a part number, I can request a successful search for all the microprocessors in RS database. Then I select the manufacturer I want etc until I have narrowed it down sufficiently to select one. Again I can download it.
However, if I gice an incomplete part number (such as 16F755 for that well known PIC microcontroller) I get the dreaded "communication error" which makes me suspect a failour of the RS search engine rather then the link itself.