It looks like their embeds need to be served over SSL now which, IIRC, when we implemented the BB code media sites in XF2, still wasn't available.
I've just updated it here and it seems to work.
For XF1, you just need to edit the BB code media site definition and change the embed HTML to:
HTML:
<iframe width="500" height="300" src="https://www.liveleak.com/ll_embed?i={$id}" frameborder="0" allowfullscreen></iframe>
For XF2, we'll ensure it's updated for the next release automatically.