I think you mean RailEurope, not Eurail. Eurail sells rail passes, not reservations. In general, RailEurope is not the least expensive way to get reservations/tickets. A better way is to go directly to the national rail sites (Bahn, SNCF).
That non-reservable connection through Frankfurt uses a TGV to Frankfurt. French Rail requires a "reservation" for passholder on the TGV. This is a surcharge for what they call a premium train. It also comes with a seat reservation, as do all TGV tickets. French Rail limits the number of passholder reservations to less than the capacity (one of the many disadvantages of a rail pass). In this case there are no passholder reservations available (at least to RailEurope) for that date, but you can still purchase full fare ticket (with reservations).
[Administrator's edit: Lee, Rail Europe does have passholder reservations available for a date two weeks from now, in both directions, on the direct Paris-Frankfurt trains.]
RailEurope (which is partially owned by French Rail) only shows connections from Paris to Frankfurt by French TGV, but if you go to the Bahn schedule website, you will find that there are also connections using German Rail ICEs. These trains also have mandatory reservations, but they are not limited for passholders. You cannot buy the reservations online, but I think you can by calling German Rail(49 1806 10 11 11 or 49 1806 99 66 33).
[Edit: Lee, I'd be surprised if "no limitations" applies to these ICE trains to/from Paris, but I'll look further. Laura]
You could also take a chance and buy them at the station in Paris when you get there. [Edit: Note that these are not sold at French train stations within three days of departure.]
"There is no such place as "Frankfort"." Not in Europe, Chris, but Frankfort is the capital of the US state of Kentucky.