Can't you download them or can't you load them?
If you can't download them from the internet, it's a problem with something blocking it, probably a Firewall, Antivirus or... I don't know, but it has nothing to do with Universe Sandbox then.
If you can't load them, I would guess that the downloaded files haven't been placed in the right folder, this might be the case if you don't see the systems in the menu. If you see them in the menu in the program but can't load them in there, that's probably not the problem.
The files should on Windows 7 be placed in:
Also shown as:
[Folder icon] ► <Username> ► Documents ► Universe Sandbox ► Systems