You'll need to edit the .map (Maps) and the .dm (DominionsMod) files in the /map and /mod directories.
Those are plain ASCII files with UNIX line feeds. If you're using Win32 you'll need an editor which can do those.
You may wannt to check out
this one.
For info on modding search the forum, there are some really helpful postings somewhere.
One word of warning: The build-in map editor just kills most entries in the map file header, so do not save any map with it unless you're shure what you're doing
