When you click 'post new topic', you will notice on your screen a bunch of buttons under the subject line. On the second row of buttons is one that has filmstrip looking picture on it. Click that, and it will prompt you to add the URL of your video.
Copy and past the URL where it prompts you after you've hit the filmstrip button, hit 'okay' and voila, you've posted a video.
For YOUTUBE videos- DO NOT use the 'share' URL. You have to go to the top of your screen to address bar and use the URL that is listed there.
If you have any questions, concerns or issues, feel free to PM me and I'll walk you through it or just post it for you if that turns out to be easier.