I had this problem the first time I tried to post, and I use a PC.
Wrong way: Having YouTube open, I right clicked on the URL, copied it, and pasted it in to my drummerworld response, like this -
Republica - Ready to Go [1997] - YouTube
Right way:
With YouTube open and with the video playing, right click on the video image itself (not the url) and click on copy url,
then paste it into your drummerworld response like this:
Then click on the drummerworld Preview button to make sure it set up correctly.