I can't make the beat extend on big band in skullgirls?

What you're looking for is a so called Shoryuken input, or a 623 (look at your numpad).

It's a bit tricky to pull off, especially for someone new. You basically want to do a movement that goes towards, down, and down-towards. I think Skullgirls is leanient with this and you can move the stick towards, down through down-towards, and back to down-towards.

Hit Punch just as you reach the down-towards position.