5th ID After Action Reports would be located in College Park if they exist (they most likely do).
I've always conducted my research via phone or mail, so I'm not sure about appointments.Check their website at http://www.archives.govhttp://www.archives.govThe site will give you phone numbers, hours, etc.
FOD means "Finding of Death".Your uncle was originally reported as missing.After a year and a day, with no reports from the Red Cross regarding POW status or no recovery of remains, the govt. would would officially declare the soldier dead.Thus the FOD.Unfortunately for many families, it happened more often than you would think.
Regarding the reconstruction unit, it's just another step in the process at NPRC in St Louis.Write back to them, enclose a copy of their reply, along with any additional information you have (even copies of envelopes from letters he sent home, if you have any). They'll send you back another form (their form # 13075) which has a more extensive questionnaire about your uncle's military service. Fill that out, send it in, and wait!!