Wayne
In MacXL the keyboard shortcut is CTRL-OPT-ENTER. To use in a formula,
use CHAR(13), e.g.:
="line 1 text" & CHAR(13) & "line 2 text"
or equivalently:
=CONCATENATE("line 1 text", CHAR(13), "line 2 text")
Make sure that the cell is set to wrap text.