Fury attacks are triggered by holding down the B button when you've acquired enough rage. Various skills will increase the rage you generate, so invest in those and trigger this often while in combat to work toward your 100 total.

Your rage icon will fill with every kill and only shows up briefly next to your life bar (looks like a bullseye) at that time. When it is completely filled, it will stay on-screen and pulse. This is when you can activate it. You can also hold RB to bring up the weapon wheel and the full HUD will display so you can check your status