Ensuring that you've got the latest version of your browser might be all that is needed to fix the problem. (As Methyl says, it would help to know exactly which browser you're using).
However I seem to remember briefly having the same problem myself a year or two ago; clicking into the box labelled 'Enter YouTube Link Here' brought up the 'Submit' button for me.
Two thoughts:
1) Were you logged in? I know it sounds obvious but we've had queries before about no option to report or select which turned out to be because the user wasn't logged in at that time
2) It may be your browser. I recall some users with an old version of IE had this issue. There are certainly some posters who don't get a submit button when they try to reply to their own questions (eg acknowledge crossword answer). If you do use an old browser try updating it to a later version of IE, Chrome or Firefox.
3) It could be something to do with her settings or maybe a lack of something like Jave or Shockwave
Since you say you typed it in the box this may not be relevant, but when it has occurred to me it is because I typed it elsewhere and then pasted it in. The lack of a keypress in the box seems to fail to activate the 'submit' button. Unsure why it has to be disabled in the first place though. Surely 'empty posts' could simply be rejected. Maybe they've had DOS attacks in the past ;-)
Sammy, the submit buttons plays up if you're running a really old version of internet explorer. Additionally, as others have said, copying in from elsewhere doesn't always register as "typed in".