copy the embed code off youtube. then click on the button that has an F inside a circle. you can find this button on the tool bar above the coment box when you post. a box will come up you copy the embed code into that box and then hit add video. then hit reply
It'd make a lot more sense if the editor would let you paste in the address of popular videos, break down the URL, and automatically put the embedded file in there. Fewer errors. Similar to what Facebook does.