I have the following code in one of my forms for the backend cms, it seems that $source will only take numeric array, not associative array.
original code:
$myMediaType = new OptionsetField(
                   $name = "MediaType",
                   $title = "Media Type",
                   $source = array(
                      "Video" => "Video",
                      "Audio" => "Audio"
                   ),
                   $value = "Video"
                  );
      $fields->addFieldToTab('Root.Content.Main', $myMediaType, 'Content');   

this will not work, but if I change the $source to a numeric array, it will set the default value.

$myMediaType = new OptionsetField(
                   $name = "MediaType",
                   $title = "Media Type",
                   $source = array("Video", "Audio"),
                   $value = 0
                  );
      $fields->addFieldToTab('Root.Content.Main', $myMediaType, 'Content');

Does anyone come cross this problem, I would prefer to store string 'Video', 'Audio' into the database rather than integer. Could someone please point me the right direction? Thank you!