How do I clean that rollerball thingy in my mouse?

A paper towel and some rubbing alcohol should be fine. Actually, the real problem with the old-fashioned rollerball mice is the three little wheels that the rollerball rolls. Those dark stripes on them are not put there by the manufacturer. They're lint that has stuck to the wheels and makes it difficult to move your mouse. I once bought a ball covered in Velcro (TM) to clean those. A Q-Tip (TM) or other cotton swab can be used as well.

Truth is, though, I usually end up scraping the dirt off the wheels with my fingernail.

on mine anyway (its old) , there is a circle thing on the bottome that twist off. when you twist it off, the ball drops out. then you can clean everything inside.