There are a few things I would try if it were my forum.
Step 1: Check other forums to do with programming, and post around the ones that let you have a signature link back to your forum. That way you are targeting people that already know the forum world and that have an interest in programming.
Step 2: Get social. Find social groups that are based around programming, and then get to know those people. Once a relationship is established to any degree, you can let them know about your forum.
Step 3: Start leveraging your current group of friends that are interested in programming. Invite them to the forum to hang out.
Step 4: Go over SEO strategies and make sure you are competing for low competition keywords that will give you a decent result. Then you can target higher keywords later on, when your forum is much larger.
Step 5: Last but not least, make sure you are posting every day on the forum. The bigger the forum, the more active it looks. Then more people are likely to join.