R20 and higher no longer have a Plugins folder in the folder Cinema 4D is installed to. Although you could simply create a plugins folder in your Cinema 4D installation folder, this is the old method and does not always work properly with R20 or higher versions of Cinema 4D.
Fortunately, in R20 and higher the plugins system has been overhauled to make using plugins a lot easier! All you have to do is set up a plugins path in Cinema 4D's preferences. Let's look at how to do that:
First, you need to create a folder anywhere on your computer* to hold all your plugins. This folder can even be on a cloud service's folder such as Dropbox, Google Drive, or whatever else you use so that you can use the same plugins folder across multiple machines! This makes things very convenient when you're working at, say, a home computer as well as a laptop or work computer. This folder can be named anything, but in this example, we'll simply call it "Plugins":
Next, You need to tell Cinema 4D where to search for plugins. To do that, simply go to the Edit menu in Cinema 4D and choose Preferences. Go to the Plugins section on the left, then click Add Folder on the bottom of this window:
This will open an Explorer (Windows) or Finder (Mac) search window which you can use to find the Plugins folder we created earlier**. Once you find and select that folder, click OK and you'll see it show up in the Search Paths box with a checkmark next to it showing that it's enabled now:
Now you can simply close Cinema 4D and re-launch it to and it should load all of the plugins installed to the new folder.
* You cannot use a plugins folder inside of your Cinema 4D installation folder for this method. Since the old method still works, folders located in the same folder where Cinema 4D itself is installed are ignored if used as a Search Path
** Note that in this step you can also create a new folder straight from the search window, so if you haven't created a plugins folder already you can do so in this step instead.