1/6/10 Updated since YT changed their page.
Mouse over the "Embed" button.
Next, if the screen you type on looks like this (gray background) then click the (a/A) as shown here to change the edit mode.
Then just paste what you copied.
You'll know you have the whole thing when you see ( < object ) at the front and ( < / object > ) at the end.
Feel free to practice embedding in this thread.