I am curious about how one would go about reading mouse data.
Well, that really depends on the type of mouse you have.
I tried using the OptiMouse library, but could not locate a mouse with the supported chipsets (that, and the project files are really picky to compile and return a weird error from math.h line 439).
Sorry. This is the Arduino forum. The psychic forum, that can divine the problem with no details, is the next web site over.
Long story short I just want to set up a program to see the raw data being returned from a USB mouse.
Is the USB mouse a USB master or a USB slave? The Arduino is not a USB master, so additional hardware and a lot of skill (and some luck) is required to make it act like a USB master, to get data from a USB slave.
If the mouse is a USB master (unlikely), then it may be able to send data to the Arduino.