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:
- Product engineering, design, digital marketing have function-specific interviews with no cases.
- Data science, agile transformation roles will require case interviews.
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.)