How do I make train reservations when I have a Eurail pass?

I am traveling from Barcelona on June 29th to Nice, France. There are only two trains leaving on the day I want to go so I want to make sure I get a seat, and reservations are compulsory, but I can't figure out how to make reservations. I've been searching online and as far as I can tell the only way to make reservations when you have a Eurail pass is at the train station, but I'll only be in Barcelona for a night and thats very short notice. I'll be in the same situation when I travel from Nice to Florence, Italy. I need to make reservations but don't know how! Help!