In the past, I can simply highlight all the cells and the sum will appear in
the right hand bottom corner. But that function is gone. Anyone knows how
to re-activate this function

Hi,
goto View in the file menu and ensure that the status bar is showing.
Then, right-click the mouse when the cursor is over the bottom right
hand corner (there are some grey boxes which are very faint) and a list
of functions such as Count, Average, Sum etc will come up. Pick the one
you want and voila!
Hth,
OJ