Go to YouTube, find a video that you'd like to link, then highlight the url (gibberish in the address bar), press 'control-C'.

Go to NitroExpressForums, compose post, click on the 'URL' tab below post (instant UBB code).
In the box that appears, delete the highlighted 'http://', then press 'control-v'. Now click the highlighted 'OK' box. If you wish to give it a title, type it in where the 'http://' had been. Now click 'OK' again.

Et voila!

YouTube