We use the following formula if you need a - between the octets:

=MID(c2,1,2)&"-"&MID(c2,3,2)&"-"&MID(c2,5,2)&"-"& MID(c2,7,2)&"-"&MID(c2,9,2)&"-"&MID(c2,11,2)

We use the following formula if you need a : between the octets:

=MID(c2,1,2)&":"&MID(c2,3,2)&":"&MID(c2,5,2)&":"& MID(c2,7,2)&":"&MID(c2,9,2)&":"&MID(c2,11,2)

Of course, you need to change the cell references in each (i.e. the c2) to match which cell you need to convert.