Step 1 - Implement a Critical Path Data Field
A new data field must be added to the Tasks entity as follows:
Note: when the field is added the Field Type must be Yes/No and the List Criteria must be Ticked.
Step 2 - Map the MSP Critical Path field to PPO
The critical path is determined in Microsoft Project (MSP) automatically and the field in MSP is called "critical":

In order for this field to be imported into PPO the MSP field needs to be mapped to the PPO field on the Task Import Field under the System Configuration:
Note: in the example above the PPO field is T20 and the MSP field is taskcritical.
Step 3 - Import the Project Plan into PPO
Import your Microsoft Project (MSP) plan into PPO. The Critical field from MSP will be imported into the PPO Data Field:
Step 4 - View Critical Path
The Critical Path can now be viewed from the Task List page:
or, by creating a filter using the Critical Path Data Field: