This is how User Model should be
class Model_User extends Useradmin_Model_User
{
protected $_has_many = array(
'plugin' => array('model' => 'plugin', 'through' => 'user_plugins'),
);
...
This is how Plugin Model should be
class Model_Plugin extends ORM
{
protected $_has_many = array(
'user' => array('model' => 'user', 'through' => 'user_plugins')
);
You don't need user_plugin Model at all, the "user_plugins" in both models refers to the table name, not the model name. Just make sure you have the table with user_plugins that have following fields,
id, user_id, plugin_id
I hope this helps.