That's actually it for both of our files. If you did it correctly, your plugin will now be accessible from Construct 2.
While we covered a bit of information, there is a lot more you could add or do in both the edittime and runtime files. I'd recommend looking through the SDK manual and looking through some of the other plugins you already have installed(Just don't make any changes or you could break them) to gain an understanding of how other plugins add different things to their plugin files.
As far as our plugin is concerned you can now use it to implement in app purchases using Facebook credits. However more needs to be done such as creating the purchase file for each item you plan on selling and setting up your company. You will also need to setup a server that Facebook can ping with information on the purchase. This is also how you would save the users purchase information for future use in your app.
If you found this useful, please like or recommend the tutorial. I apoligize for skimming through some areas. I did not want to bog anyone down. Read the SDK manual and learn the skimmed over areas at your own pace. Or learn by looking through other plugins.
I'm available for projects, so if you need assistance on your app or creating a plugin, please feel free to send me a private message. I would be more than happy to discuss helping you on your projects.