Where can I find the "50 additional Integrated Reasoning questions"?

I may not be the best person to answer your query.

Just try to take help from help page below og wiley
https://gmat.wiley.com/index/login?page=login#

If needed reach out to them on their Help and Support Team

Thank you for contacting Wiley Technical Support.

The GWhiz Mobile site is actually a long out-of-date site for an older edition of the book. For the 2017 edition of the GMAT book, the IR questions were combined with the rest of the questions available at gmat.wiley.com.

To access the IR questions, simply log in at gmat.wiley.com and under either Practice or Exam mode, you'll see the various categories you can select and de-select. At the bottom of each list are the IR question categories. If you'd like to only practice the IR question content, de-select all other options and begin the exam.

Please let me know if you have any further questions or concerns!

Wiley Technical Support