We've had numerous users report that the hyperlink button in the add/edit page/post screen is grayed out and not working.
I'm creating a sticky topic on this at the top of the forums so that other users who come here will see it know we're aware of the issue and working it. It's one of the highest priority items for the next update, hopefully a week or less away. If we find a patch, we'll also post it here.
Most users are not having this problem, so we need to be able to diagnose what's producing it for specific users.
For that reason, if you are having this issue, I'd like to ask if you'd consider submitting a bug report with login credentials. We'd need WP admin credentials at the least, and if you want to add FTP, we'll patch your site with the fix when we find it.
Otherwise, you are welcome to post any other details here and we'll see them. You can follow/subscribe to this post and you'll get updates.