You’re right about the ‘previous post’ and ‘next post’ being missing from the language file. Thanks for the report! I’ve corrected them.
You API key should have been sent together with your purchase receipt. If not, you can get your API key from this page: http://www.khairul-syahir.com/products-page/your-account
Login and then click on the “Downloads” link, and you will have a list of the downloadable products you’ve purchased and the corresponding API key.
Let me know if you have any problems.