Open User Settings -> Account Integrations to make sure you're connected to the correct version control provider.
Click Configure to make sure expected repositories have been connected to PullRequest.
For more information, refer to our integration guides:
Log in to PullRequest and select the appropriate GitHub Organization from the selector at the top of the screen. Then select Pull Requests from the sidebar.
Click the Send to PullRequest button on any of the code reviews you need to assign to the network to get reviewed.
The status will change to "Sourcing Reviewers."