First of all, you should ask the HR in detail what to expect on the interview. McKinsey Digital labs has various roles. Some of these roles require a case interview. Based on the experience of my candidates and friends working in Mck Digital:
In the second option, you should prepare in the same way as for regular McKinsey interviews, plus have relevant expertise in a particular field. For example, I've seen many agile transformation cases for these roles.
For the regular McKinsey cases preplounge will be most helpful. I also recommend reading McKinsey articles about digital on it's website (Hundreds of them if you google by key words "agile", "Digital", etc.)
If you are applying for the 'consulting' track (vs. specialist such as data scientist or program developer), a similar question has been already addressed in this thread:
The same rule applies to McKinsey.
On the other side, if you are applying for a specialist position (e.g. data scientist) the interview will test as well your technical expertise; unfortunately I do not have much visibility on it.
Feel free to reach me if you have any additional question,
I also applied the similar role and for agile transformation, now in round 2 interview phase.
So cheers and finger crossed... haha!