The DNG converter is not a plug-in. It is a standalone program.
I don't know why, but Adobe archives all old versions of DNG Converter on its website. In reality, there's no reason I know of why you shouldn't always use the latest one. The current version is 8.2, and is available here: http://www.adobe.com/uk/products/photoshop/extend.displayTab2.html#downloads
As Jim said, it's a stand-alone program. Assuming it downloaded without error, you execute it to install it. The installation extracts the program file itself, as well as supporting camera/lens profiles. I'm not sure how this works on the Mac.
To use the program, you can either open it and pick a folder of raw files, or you can drop a folder onto the program icon.
If you are using an old version of Photoshop, you may need to change the Compatibility preference from within DNG Converter to match the version of Camera Raw in use. Always use the latest version of Camera Raw available for your version of Creative Suite (e.g. 4.6 for CS3, 5.7 for CS4, 6.7 for CS5, 8.2 for CS6).