How to add a line break in a cell in Excel for Mac
The answer is to use Control+Option+Enter, which will create a line break in the cell.

As an alternative, Control+Command+Enter can also be used to achieve the same.

These combinations work both in Excel for Mac 2011 and the new Excel for Mac 2015.

As pointed out by Shameer in this answer, Alt+Enter now seems to work as well in the new Excel for Mac 2015.