To make the box for the matrix, take a look at my previous instructable.
Here's a video demonstration the end result:
I made this project over at TechShop (http://techshop.ws/).
Step 1: Materials/tools
16x24 HT1632C LED Matrix
Wire/ hook-up wires
Step 2: Wiring everything up
Step 3: Upload the program
The code's really long so I won't paste the whole thing here, I've attached it as a file to download and plug into Arduino. I uploaded two variations: one that plays the alphabet sequentially and another that cycles random letters. The code's a ton of copy/paste and isn't too complicated, but if anyone has any specific questions I'll be happy to answer them in the comments.
And that's all she wrote! There's a video of it in action in the intro of the instructable. Thanks for reading!