To grab yours, start up the game and either log into your existing account or create a new one. Make sure you're playing the character whom you want to receive the weapons because you can only grab these once per account! Zip through the tutorial missions and then head to the Spacetime Nexus from the main lobby screen. There, on the bottom right of your screen, you'll see a button for enterting a promo code. Enter the code massively20120410 and find your maces waiting for you in your character's inventory.
The code has unlimited uses, so everyone who wants one can use it, but do note that it will expire eventually, so snag that gift sooner rather than later. Many thanks to Spacetime Studios -- have fun!