Heraldry-Ordinary question

[MacMillan of Rathdown]

Here is my attempt at creating something for my family in NZ.

What you have is a logotype, but not a coat of arms. Now if you were to place this logotype within a single continuous line shaped like a shield you would have a coat of arms because it could be blazoned (the primary test). I should also add that it would be a rather... ah hem... poor example of heraldry.

Placing the shakefork between the southern cross is okay. If you feel the need to include the fern, the best choice would be to place it on a sable chief. Finally, whilst scrolls and words do appear on shields from time to time, to the best of my knowledge, they never repeat the family name. That being the case, I'd advise you to loose the scroll with your family name on it.

So, you might consider as a devisal:

Argent within a representation of the Southern Cross constellation a shakefork sable on a chief of the last a silver fern leaf fess ways proper.

There is no need to place the family name on a scroll under the shield, because the shield itself "says" the family name.