There's no reasons why you shouldn't be able to download from the site which hosts the FSX version (which is a different one than the FS9 version), the site doesn't have any access restrictions.
Instead, the site which hosts the FS9 version, since it also hosts this forum, to protect from spammers, has a lot of IP from some countries like China blocked so I would expect you'd might get that error from the FS9 site instead.
The two sites are entirely separate, don't have anything in common, and are administered by different people, the one with the FSX version is in the US, the one with the FS9 version is in Italy so, the only explanation I have, is that something on your end is wrong, also considering nobody ever reported a problem with the Stand-Alone Addon Manager download until now.
It happened once, a customer told me he wasn't able to access the virtualisoftware.com website, because some corporate firewalls block it, since it's related to games, and there are policies which blocks everything like from videogames, to casino games, etc...
