Use this script in "P"-Mode. It will take n pictures at -2,-1,0,+1,+2. Kinda slow but it works for me. I use the CHDK build from here
@title HDR @param a Number of shots per step @default a 2 let i = 1 print "Preparing" click "expo_corr" for n = 1 to 12 wheel_left sleep 100 next n for m = 1 to 5 for n = 1 to a print "Shoot", i, "of", a*5 shoot i = i + 1 next n for n = 1 to 3 wheel_right sleep 100 next n next m print "Resetting" for n = 1 to 6 wheel_left sleep 100 next n end