I posted in the Royal arms thread about this. The problem is that the Royal Arms rar file does not have the correct directories in it.
It has:
Royal Arms\data\base
Royal Arms\data\imperial_campaign
where it should be
Royal Arms\data\world\maps\base
Royal Arms\data\world\maps\campaign\imperial_campaign
I hope Xerces can fix this.
Rothgar - you could unzip the rar file to a temporary directory, and then copy
"Royal Arms\data\base" to "Medieval II Total War\data\world\maps\base"
"Royal Arms\data\imperial_campaign" to "Medieval II Total War\data\world\maps\campaign\imperial_campaign"
Or if you are using MedManager, create a "Royal Arms" mod icon first, and then copy
"Royal Arms\data\base" to "Medieval II Total War\Royal Arms\data\world\maps\base"
"Royal Arms\data\imperial_campaign" to "Medieval II Total War\Royal Arms\data\world\maps\campaign\imperial_campaign"