I've been through the transformation into a werewolf with the companions but know do i just equip the power or do i have to press a key because i've equiped the power and i'm not turning into one.
To equip it, you need to go Character Menu > Magic > Greater Powers > Werewolf power and select it. (A little white arrow should appear next to it on the left). This will equip the power.
To use the power, it's the same button to use shouts.
2 Werewolf notes:
*The exception is to do a deadric quest and get the Ring of Hircine, to allow more transformations per day.