That sounds like a strange issue, I have WordPress 3.5 running myself and I'm not encountering this type of behavior in Chrome, FireFox, or IE.
You should also be able to click on the Text editor tab instead of Visual up at the top right of your post and manually add a link with syntax like this:
I did some searching for you and ran across WordPress support forums - 3.5 Broke "Create Link" button and "Text" tab in post editor and it seems other people are having similar issues depending on the theme they're using.
You could try to revert back to a default theme and see if you're still having issues. It was also mentioned that adding the following lines to the bottom of your wp-config.php file can resolve this problem in some cases:
define('CONCATENATE_SCRIPTS', false );
Hope that helps you out, if that did work for you please let us know so that we can create an article about this to help out other WordPress users.
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