How do I preserve leading zeroes in Mac Excel?
From the Home tab, select the cells you wish to format, and from the format drop-down under Number select Custom. Or, right-click and choose Format Cells, or hit Cmd-1.

This will pop-up a dialog window with the various formatting options.

Again, make sure you're on Custom.

In the Type box type in: 000000

Click Ok.

Alternatively - change the formatting type to text. This will keep whatever you type in the box.