Copy tinymce_setup.js (/media/tinymce_setup/tinymce_setup.js) in order to not break future updates and adjust the setup/behaviour of TinyMCE according to your needs - see http://wiki.moxiecode.com/index.php/TinyMCE:Configuration. Please note that tinymce_setup.js is just an example. You probably need to change the setup-file.

Change the media-definition of your ModelAdmin

    class Media:
        js = ['/media/admin/tinymce/jscripts/tiny_mce/tiny_mce.js', '/path/to/your/tinymce_setup.js',]