Restore Health:
1) Whilst in the game, press B to bring up the menus.
2) Use the L and R triggers to switch to the Stats menu.
3) Use the D-pad to select the Health entry.
4) Press Black, White, Black, Black and Black.
5) Press and hold A until you have reached the desired Health level. 

Restore Magicka:
1) Whilst in the game, press B to bring up the menus.
2) Use the L and R triggers to switch to the Stats menu.
3) Use the D-pad to select the Magicka entry.
4) Press Black, White, White, Black and White.
5) Press and hold A until you have reached the desired Magicka level.