After setting up your 360 Feedback cycle, to manage nominations you'll need to visit the Company Administration page for the cycle. Using the 'manage reviewers' admin option, you'll be able to manage or approve reviewers.
Manage Reviewers from the Company Overview
Visit Company -> 360s -> Cycle Administration page -> Manage Reviewers:
You'll be able to add new reviewers, approve all reviewers, and remove reviewers if needed. Below is an example.