To embed a video from nhl.com, like this:
1) Go to a video on nhl.com.
2) There is a drop down shade at the top of the video that gives you a share icon (three dots connected by lines). Click on it.
3) The 2nd field should be labled "embed code" that looks like this:
<iframe src="http://video.nhl.com/videocenter/embed?playlist=750019" frameborder="0" width="640" height="395"></iframe>
It's important you use the "embed code" link and not the "url" link...they are different.
4) Just copy the portion of the link after 'videocenter' and until the closing quote on the url. So in the case above, you would copy this portion:
(and don't forget the '/'...and make sure there are no spaces before or after the pasted link, as it will insert them in the bbcode and it won't work)
5) Within the post editor here, click on the "nhlvideo" button and paste what you copied inbetween the nhlvideo tags.
It should then look something like this: