Connecting to Marketo requires you to provide three pieces of information as seen in the screen below.
1. REST Endpoint
2. Custom Service client ID
3. Custom Service client secret
For REST API endpoint, go to Admin->Integration->Web Services and then scroll down to the bottom to get to the REST API section.
For 2 and 3. Click on the Admin->Integration->LaunchPoint. Click on view details for the REST Service.