its quite simple actually. alls you have to do is go to your /data/maps/campaign/imperial_campaign folder. and open up the descr_strat. you should see something like this
imperial_campaign
playable
romans_julii
romans_brutii
romans_scipii
end
unlockable
egypt
seleucid
carthage
parthia
gauls
germans
britons
greek_cities
end
nonplayable
romans_senate
macedon
pontus
armenia
dacia
scythia
spain
thrace
numidia
slave
end
now just move macedon from under nonplayable to under the playable heading like this
imperial_campaign
playable
romans_julii
romans_brutii
romans_scipii
macedon
end
unlockable
egypt
seleucid
carthage
parthia
gauls
germans
britons
greek_cities
end
nonplayable
romans_senate

pontus
armenia
dacia
scythia
spain
thrace
numidia
slave
end
thats all there is to it.