The Sienna fuse boxes are located in the engine compartment on the right side looking in from the headlights and radiator. In the interior of the Sienna the fuse boxes are located driver's side lower trim panel ( I forget if you have to remove a cover on that trim panel to the lower right of the steering column as viewed sitting in the driver's seat or whether the fuses are accessed kneeling on the ground looking up under the dash. There are two fuses just behind the glove box on the passenger side again I forget if you have to remove the glove box or gain access to these fuses from under just under the dash. The two fuses located by the glove box are for Air Conditioning and the Starter circuit.
The interior fuse panels are typically located at the lower left of the dash. Sometimes they are near the driver's side kick panel (to the left of the pedals). Toyota usually has good information in the owner's manual about fuses and light bulbs. I would check that if you don't see it below the dash or by the kick panel. Sometimes there is a plastic interior panel that pops off to reveal the fuses.
Do these pictures help?