TRISTLW
If I work a full time job but I also have an internet start-up. How would I add that to my resume??
Just include it like you would any other employment. If your full-time job is more relevant to your MBA app, I'd put that as the first item under your Experience section, but then list your company and have "Founder" or whatever you like as your position.
I started a small business while employed full-time, but I had a co-founder who was responsible for more of the day-to-day operations. I later left my full-time job to join the business full-time. On my resume I show 2 positions at the start-up business -- the period that overlaps with my full-time job my position is "Co-Founder and Strategic Advisor" and I talked about my involvement in the business during that period, so it made sense that I could do it in my time outside of my day gig.
I think the important thing to do is make sure doing both is at the same time is believable to the adcom reader, but don't undersell your start-up.