Find the file descr_strat in C:\Program Files\Activision\Rome-Total War\Data\World\maps\campaign\imperial_campaign
(or which ever folder you installed to)

at the top of the file cut and paste the word macedon from the unlockable section into the playable section. Make sure you've got the spacing correct and save the file.

Bingo! Macedon are now playable!

Code:
; Custom campaign script generated by Romans Campaign Map Editor

campaign		imperial_campaign
playable
	romans_julii
	romans_brutii
	romans_scipii
	macedon
end