For those who don't know. Go to your Tools pull down menu and select Options. When the Options dialog opens, click on the Files tab. In the Search Paths window, double-click on Printer Support File Path and then double-click on Plot Style Table Search Path. This will show you where your ctb files are stored.
You can then copy and paste, any ctb files you receive, into that directory and they will show up in your Plot Style Table in the Plot Dialog.