mapedit.pdf can be found in your dominions3\doc folder.
I highly recommend spending some time with this document and feel free to come back with any parts that you need help with.
It is of note that Edi is working on a Map Editing Manual that should prove to be a great resource when he is finished. The thread about it can be found here:
http://www.shrapnelcommunity.com/thr...&Number=620966
The relevant command for placing nation specific starts:
#specstart <nation nbr> <province nbr>
Use this command to assign a specific nation to a specific
start location. Nation nbrs can be found in the modding
manual.