You should now see two batteries, and one small piece of metal which separates the batteries. Save this piece of metal which separates the batteries - it must go back in with the new batteries, in the same relative position it was in when you removed it (with the tension tabs facing the proper direction).
Insert the next battery, positioning it just like the first one - negative end facing toward the first battery, against the metal tensioner. Check to make sure the batteries are inserted all the way.
Place the plastic cap over the open end of the battery case and snap it into place. This is easily done by placing the battery case on a table top, with the open end facing up. Place the plastic cap over the open end of the metal case, and press down firmly and evenly. The cap should lock with a "snap" onto the metal end of the battery case.
The batteries should now be electrically checked by inserting them into your backup pump - insert a battery into the #3 compartment first, then insert a battery into the #2 compartment. All pump functions should work fine. If a battery alarm does occur, it is usually because the tensioner piece was inserted in the wrong direction, or because the polarity of the batteries is wrong. Open the battery case to make sure the tabs on the tension piece are facing the plastic cap on the battery case, and check to ensure the batteries are positioned properly (negative side facing away from the plastic end cap).