There's a basic HTML tag for it, but MKO doesn't allow embedding of youtube videos. Out of consideration given to those with slow connections, they try not to make it too hard for people to surf the site

Vash beat me to it but, yeah, you can't

avatars and sigs don't show up for me though except that top left section with users and inbox

that's because you were shadow banned, if you want it lifted, PM a mod asking them to lift it, and maybe ask why you were banned in the first place.

oh, and use your manners

??? interesting, had no idea they could do that anyway

It happens if

A) Your sig/avatar are offensive

or

B) Their file size goes over the allowed limit

Vash basically covered it; no embedded videos in signatures.
Jason, your signature and avatar must have a combined limit of less than 250 kb. If you're above the limit, then you are shadowed by moderators like myself. Feel free to message us when you make your avatar and/or signature smaller.