The deployable ballast is a quest item given to the Courier by Loyal at Nellis Air Force Base. It attaches to the crashed B-29, one ballast on the underside of each wing, between the two engines (the location is indicated by a transparent ghostly outline). Once detonated, the plane is able to float on the ballasts.
During Volare!, they have to be put under each wing on the B-29 as stated above. Lake Mead is a rad-free but very deep lake, so you need to talk to Loyal about getting to the plane safely if you don't feel like chancing it. He will send you to his assistant, Jack. Jack will say he can help you, but in order to do that, you need to either bring him a pressure cooker or pass a science check of 45. If you want to find a pressure cooker quickly, there should be one in the kitchens at Nellis AFB. Once you have the cooker, he will give you the Rebreather, which allows you to breathe underwater. Attach the ballast and complete the quest by traveling to the quest marker, on the shoreline. Loyal's detonator will start to beep, sounding like a land mine. Equip Loyal's detonator (it is in the weapons section) and fire it.
pc If the player attaches the ballasts on the plane and then goes back to Loyal and talks about the plane, Loyal will give the player two additional ballasts, which can't be removed from one's inventory even after the quest (because they'll still be marked as quest items). This can only be fixed with console commands. [verified]