I just can't figure out how you actually assign a perk. what key is used?

You click the left mouse button.

Use the A and D keys to scroll though the skill list, then you just select the perk with your mouse. Remember you do need a certain level of skill before you can select a perk and you have to follow the perk tree, some perkes are needed to unlock the next etc.

It's the left mouse button (or WASD) to move through the perks and E to take one.