<p>I really can't find another place for it, but it's very important to me and I devoted a lot of time to it, so I don't want to leave it out of my application.</p>
<p>I would say yes, actually.</p>
<p>The questions on the MIT application are generally there to be useful to you, and it’s up to your discretion how you’d like to fit your real, messy, category-crossing life into neat boxes on the application. In your case, if you have an activity that looks like employment in every way except for a paycheck, I say feel free to call it employment.</p>
<p>“Pro bono”? ;)</p>
<p>^Right? 
I know I’ve done some things in my life for free that I took as seriously as a job, and some things for pay that I would have taken equally seriously if I were doing it for free (e.g., I got paid to blog, and I don’t get paid to mod on CC…).</p>