IF you are talking about a worksheet rather than a user form, you can make it
tab from unprotected cell to unprotected cell by clearing the box next to the
"Select Locked Cells" option when you Protect the sheet. Tab or Enter will
then take you to the next unlocked cell.
Whether the next move is to the right, left, up or down is controlled by
Options, [Edit] tab, "Move direction after Enter" option on that tab.