I was out on a ride with someone once and noticed that the pin was in the wrong place so I told him that he had fitted the ring in the wrong orientation.If it's the pin between the crank and the ring, then it's to prevent it getting jammed.
This is to help the chain ride up from the smaller chainring to the larger one. There should be others around the chainring maybe in 3 or 4 places? I don't think you can get pins separately. Is the changing being noticeably hindered by the absence of this 1 pin? If not, ignore it and carry onIt was the pin attached to the large chainring and facing in towards the small chainring. Can replacement pins be obtained and fitted?