That looks like your oil drain plug. My '95 has a hex head bolt in that position, but yours looks like a plug with a small Allen head (hex) hole for an Allen wrench. In other words, I don't see that anything is missing. If you dry all the oil off carefully and watch it, can you see exactly what it's seeping from? Just going by the photo, it looks like the oil filter cover might be leaking a little.
After you dry it off, you can puff some talcum powder on the surfaces to make tiny amounts of oil show up quickly.
I noticed that my copy of the shop manual has the labels for the oil filter bolt and the drain plug reversed, but it's labeled correctly here:
http://faq.ninja250.org/images/8/84/2Winterize.jpg