there are 5 files that need to be edited:
HTMLEditor/custom/contents_insert_ContentValue1.js
HTMLEditor/custom/contents_insert_ContentValue1.php
HTMLEditor/custom/contents_update_ContentValue1.js
HTMLEditor/custom/contents_update_ContentValue1.php
HTMLEditor/fckconfig.js
in all 5 of those files, find the line:
FCKConfig.EnterMode = 'p' ;
and change it to:
FCKConfig.EnterMode = 'br' ;