Enabling MFA for a user is done in the dashboard by clicking your account name (top right) and going to profile.
To enforce MFA for the organization, go to Organization -> Settings.
As for logging, all event logging for a given network is under Network-Wide -> Event Log.
You can then use filters there to narrow down what you're looking for.
If you have a specific client in mind, you can also select the client from Network-Wide -> clients and click "event log" or "timeline".
Edit: It just occurred to me you we looking specifically at client VPN.
Meraki client VPN doesn't natively support MFA but can incorporate 3rd party solutions
Two-Factor Authentication - Cisco Meraki
The above info about logging is still applicable for VPN connections, you would just filter to event type.