Check the version of the plug-in you have, if it is compatible with the latest Aperture version. With Aperture 3.4.3 and Mt. Lion all plug-ins must be sand-boxed, see: Aperture 3.4 and later: Some third-party plug-ins are no longer compatible
If your plug-in is compatible with Aperture 3.4, it should be installed in the Applications Support folder in your User Library or System Library:
either in the sub folders "Export" or "Edit".