On your MainWP Dashboard site:
- Use the Security Ninja item in the MainWP left menu (opens the global events log), or
- Go to Extensions → Security Ninja for MainWP
If the overview is empty, finish setup first: connect child sites, install Security Ninja on those sites, then sync. Full steps: Install Security Ninja with MainWP.
Also useful: Get started with MainWP, Install the free MainWP extension.