The high-level things you need to do are:
1. Create a dashboard account and add the device SN# to the inventory.
NOTE: If the previous owner has not released the device from their dashboard account, you are stuck. There is no way you will be able to use the device.
2. Create a network in the dashboard and add the device to the network
3. Purchase an MX67 license and add it to your dashboard. If you don't purchase a license, the device will disconnect from the dashboard in 30 days.
There's plenty of guides and docs for getting started:
Getting Started with Meraki - Cisco Meraki Documentation
Getting Started Checklist - Cisco Meraki Documentation