I am aware that the website does not directly support the 6th edition of Remembering the Kanji. For the time being gomtuu's 6th edition frame number converter can be used to add flashcards in order for the 6th edition index (forum thread).

When using the Study page, keep in mind you can search by keywords and also by character. While the index is different, most keywords remain the same between 5th and 6th edition. ALL characters in the 6th edition are present on the site. They are just showing under a different index number (many characters that were present in RTK Volume 3, were added to the 6th edition to keep up with the official "common" kanji).

Admittedly this is a cumbersome way of navigating through the Study pages since the previous/next arrows advance using the 5th edition ordering. I will try to get 5th and 6th edition support (both) working before the end of December.