Make sure the GitLab PullRequest app has been added to the GitLab organization that contains the repositories you require for code review.
You can verify PullRequest has been installed and added by checking here.
From the PullRequest dashboard select the GitLab Group and Project from the selectors at the top of the screen and select Reviews from the side menu.
Click the Request Review button on any of the code reviews you need to assign to PullRequest.
Once assigned, the pull request's status will change to Review job created. Once a reviewer has been selected and has started work, the status will change to Review in progress. Once a PullRequest reviewer has submitted a review, the status will change to Review complete.