How do you attack in arena? I tried left clicking but
nothing happened.

Have you waited until the gates open and the match actually starts?

Hold down the left mouse button and move your mouse across the screen. In Arena and Daggerfall, players must swing their weapons by holding down the attack button and dragging it across the screen in various directions.

Thanks for the info. also how do you loot a corpse?

Click on it.

Tried that nothing happened.

Did any messages pop up?