The newly redesigned battery compartment on the MSA/Sordin headsets is very snug as to limit any movement and thereby a shut down of the electronics.  Place the first battery in positive tip out (toward you) and tap the bottom of the set on a hard surface this should allow the front of the battery to drop into place. As the new boxes are very tight for the first few sets of batteries (several years worth) if tapping the headset does not seat the battery far enough to insert the second battery (which goes in positive tip in) then gently insert the blunt end of a pencil (careful not to go all the way to the back of the compartment) and push the rear of the first battery down until you feel it click into place. You should now be able to insert the second battery into place.  When removing the batteries it is often necessary to turn the headset upside down and curtly rap it on a hard surface to dislodge the lower (first) battery.