I had the same issue a few days ago. They recently implemented some nonsense to block users from accessing their site that use adblockers or hosts files to block ads, resulting in the error you get.
I love how they start doing this only a week or so after they infected thousands of users with a virus via a flash exploit from one of their shady advertising partners. They can go screw themselves IMO.
Clicking this link should let you download the client, which will still work regardless. You can also search for new addons through it. I'd suggest using a new site to read about addons you wish to install such as wowinterface.
http://clientupdate.curse.com/setup.exe