[eluser]mddd[/eluser]
In plugins en helpers, you must first get a reference to the CodeIgniter object. Then you can call that object in stead of '$this'.
Code:
$CI =& get_instance();
$CI->mymodel->myfunction();
However helpers and plugins are mostly meant for providing simple functions that don't need to reference models etc.
If you need access to your model, you might as well put the "plugin" function inside your model!