My WordPress link button was perpetually grayed out / not clickable. And no, it wasn’t because I wasn’t highlighting the text first and THEN clicking it. The link button flat-out didn’t work in WYSIWYG (it worked in Text-editor / HTML though).
I had to revert back to a pre-2.0 version of NGG in order for my link button to work in WordPress 3.6. I am using Tiny MCE but disabling or enabling that plugin had no effect, only disabling NGG 2.0 made my link button clickable again.