I purchased the Nippon DLC for Magicka, but I don't know how to 'equip' items or to change the items my character already has.
How do I equip the new robe, katana, and staff?
I didn't buy it yet, but it should be the same as with all the other DLC packs.
When you get to the lobby screen that's present on both single and multi player, click on your character, which should bring up this screen:
Use the tiny arrows to cycle through your available characters.
You will spawn (and respawn, in multi player mode) with the items the character is holding; however, if you're playing a save in progress, you will retain the weapon and staff you had last.