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".
You may have been using the plug-in called: "Catapult" at link below:
As Leonie mentions, you will need to verify compatibility with the new sand-boxing requirements if using Aperture 3.4.x. I suggest contacting the author(s) of Catapult to verify.
Note - I have never used Catapult, but do use DxO Optics Pro 8 before importing into Aperture for a kit lense that needs optical corrections beyond the capabilities of Aperture (and takes too much time in ACR).