I just purchased "Prem" tickets from TGV-europe for 4 people at a great price, received my e-ticket confirmation, but was unable to reserve seats. Does that mean we will not have seats on the train or do I need to do something else to get seats assigned prior to our departure. We are not flexible in our travel arrangements, as we have a full day private tour arranged in Carentan for that day and need to arrive/depart on time.
You were not assigned seats because you will be on Intercites trains which do not require seat reservations. I doubt that the trains you will be taking are likely to be full. However, if you will be in Paris a few days before you go to Normandy, you could go to any SNCF station and ask. According to eurail.com, it is possible to buy seat reservations for Intercites trains.