On a freshly installed page, there is no InterWikiMap page. When the user comes to edit it, there is no indication about its proper syntax.
I propose shipping a default InterWikiMap file with MoinMoin, containing something like this:
#format plain # # Add InterWiki entries, one per line, in the following format: # # name url # # for instance: # # google http://google.com/search?q= # UPC http://www.upcdatabase.com/item/