After creating a lookup relation on an MDA Data - Object, in the rules engine you can select the MDA Data - Object to run a rule.

Note: Before performing MDA Joins in Rules Engine, you must create a lookup relation on an MDA Data - Object.

The following image highlights how various collections such as Account and Associated User are linked to Joins usage.

Note: You will be provided with only 2 sub-level objects containing lookup relation. For example, in the following image Joins Usage -> Account -> Active User, where Joins Usage has a lookup relation on Account, and Account has a lookup relation on Active User. However, you cannot have an MDA Data - Object containing lookup relation at the 3rd sub-level, such as Joins Usage -> Account -> Active User -> [Another Collection Name containing lookup relation].  

Sample Use Case

You might want to fetch Account and Count of high priority cases generated over the last 7 days for specific industries.

In this rule on MDA, Account has a lookup relation with Case.