If you just added the devices to RMS for the first time, they will automatically start using their free credits (each device gets a free 30-day credit when it is first registered.
If you bought credits and want to start using them, it depends how you got them. If they were just added to your company as a pool, you can activate credit usage in the SERVICES → DEVICE MONITORING page. To make a device start using a credit, turn the SERVICE slider to 'on'.
If you received a special code, you need to activate that code to get your credits. Go to SERVICES → CREDIT POOLS, hover on the CREDIT POOLS drop-down and click 'Activate code'.
Some more info can be found here.