Hi you can use TGV services CDG Airport to Brussel South (Midi/Zuid) (you need a reservation) and from there the first IC train you can catch to Leuven.

Also possible to use more frequent running Thalys trains from Paris Nord station (RER train CDG-Paris Nord not included in the pass) to Brussel South (midi/zuid). Thalys needs a reservation.

Also possible is to use TGV services to Lille and from Lille Flanders local trains to Belgium. 

Here a Tutorial I made where you can get the reservations for this services.

I suppose that "CDG” is Aéroport Charles de Gaulle near Paris. I'm not sure why you would think you can’t use your pass from there. Many TGVs serve that station (also to Brussels) and the pass is valid in them. But you need to make a reservation. You can do that here:

https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish

There are also suburban trains (RER) to Paris Nord and on those trains, the pass is not valid.

Thalys from Paris Nord to brussels can be booked here:

https://www.b-europe.com/EN/Booking/Pass#TravelWish

The best is to leave enough time between your flight and the train. If you arrive earlier, you can maybe change the TGV reservation to an earlier train (if there are pass holder seats available). The other way around, it doesn't work so well because you can't change a TGV reservation anymore after the departure time.

Also take into account that pass holder seats on Thalys and international TGVs are limited. There are also reservation-free options, e.g. via Lille or via Maubeuge.

Finally, if Charles de Gaulle airport to Leuven is your only journey, then a pass will not pay off.