Open the box and leave it in the bathroom on top of a shelf or cabinet for a couple weeks.

The steam and moisture from taking showers and baths will put some moisture back into the wood. It's easy to do and takes no effort on your part. It's a slow process, and will work perfectly if you just leave it alone for a few weeks. Trying to mist the balsa with a spray nozzle or even dunking the balsa in water is the "I'm in a rush" way of doing it and you won't be near as happy with the results.

It took 15yrs for the wood to dry out. Do you really want to put all that moisture back into it in 5 minutes? Stick it in the bathroom.