I am trying to combine fields in Excel 2007

=CONCATENATE(AB4,AC4) or
=AB4 & AC4

and when I click enter and move to the next field the field where I left the
formula remains =CONCATENATE(AB4,AC4) and doesn't seem to exicute. I was
able to make a single cell reference bring forward the value. Example =AB4
and enter then the value of AB4 appears in the cell. Frustrated Thursday.