I don't know about addons that simulate Mercury or Gemini for Orbiter2010, but you couldn't go wrong by going to NASA's website and diving into it's archives.
NASA's History Series is quite good. It has a fascinating book available about Gemini, called "On the Shoulders of Titans". I enjoyed it, but that might not tell you that much. Most of the books make for dull reading, but you can learn so much if you can keep your eyes open. There isn't a good one for Mercury though.
Wkipedia the particular mission your thinking of and go though the references and external links and follow them to the sources! Wikipedia may not count as a good source, but it often has links to places which can be very good sources. You have to dig.
Welcome to the forum!
Edit: I forgot to point out, there is lot of Mercury/Gemini/Apollo(especially Apollo)stuff on various other parts of the websites NASA maintains. And I just got a idea; try searching in Google for media kits for the programs. Since they are made for relatively un-technical people the should have nice graphs and pictures illustrating parts of the missions. Though that may only be profitable about Apollo, given that NASA's openness to the media came most after Gemini.
Edit2: You could get your hands on a copy of Orbiter 2006P1, and then you can use the old [ame="http://orbithangar.com/searchid.php?ID=1878"]Project Mercury[/ame] and [ame="http://orbithangar.com/searchid.php?ID=1450"]Project Gemini[/ame] addons. They both work with Orbiter2006P1, though you could download them and hope they work in Orbiter2010. Hope this helps.
