Should I get post-bacc in Mathematics or go straight to Computer Science Research? I have just been hired to work in the IT Division at a top 10 University in the U.S., and they said that I can take classes at the college.

Currently, I have a B.S. in computer science and a minor in mathematics.  I'd like to do research in machine learning, and its application to computer security in graduate school.  I have a very cool idea for a research topic, which appears that no one else has written papers on.  

There are two caveats to this.  One is I only have a 3.0 GPA, which is required for acceptance into graduate school.  Second, the mathematics required for machine learning seems to go beyond what I learned for my math minor.  Advanced statistical concepts, such as Hidden Markov Chains, which I've noticed is a topic covered in graduate level mathematics courses.

Should I go for a post-bacc in applied mathematics?  Should I teach myself the advanced math topics, and try to raise my GPA by taking some higher level undergrad classes in Computer Science?  

Any guidance or suggestions are greatly appreciated.
Tough question, mostly because it seems it would have a lot to do with you and your overarching goals.  Is the goal to advance your career or is your goal to get the Master's degree and write that paper and publish?

It's tempting because you have the free course option right in front of you but you can take classes in machine learning from Coursera (for instance), for not much more. So if your goal is simply to move your career ahead school is likely not required at all.

But if you want to teach and publish and get that piece of paper then absolutely you'll need more school. Take undergraduate courses and get that GPA up and then apply to grad school.