As described here: http://community.impresscms.org/modules/newbb/viewtopic.php?post_id=49033#forumpost49033

To add the options to set "Left" and "Right" as choices for the default view for the forums, you will need to edit modules/{iforum directory}/icms_version.php and add them to the config item.

$modversion['config'][] = array( 
'name' => 'view_mode', 
    'title' => '_MI_VIEWMODE', 
    'description' => '_MI_VIEWMODE_DESC', 
    'formtype' => 'select', 
    'valuetype' => 'int', 
    'default' => 1, 
    'options' => array(_NONE => 0, _FLAT => 1, _THREADED => 2, _MI_COMPACT => 3, _LEFT => 4, _RIGHT => 5));

After you save this, update the module, then set the view mode in the module's preference ("View Mode of the Threads"). Clear your cookies and view the pages.