Some plugins come with installers and load into system locations. Generally plugins are manually installed in Motion Templates in the Movies folder. All that I've ever seen get accessed from from the various browsers, effects, transitions, titles, generators.
The adjustment layer web page says install in Motion Templates. This is the typical folder structure inside Movies.:

One other factor is the folder names, which depends on whether or not your have the Motion application installed. If you have Motion, you just load them as above. If you don't have Motion you have to add the extension .localized to the Motion Templates folder and to each of the browser name folders you use, Effects, Generators, Titles, and Transitions. The further embedded folders, like the category folder and the plugin folder, don't need to be .localized. Any plugin using this structure has to be in a category folder, either a custom named category or an existing category.