Can you please define what you mean by "current bandwidth usage"? Are you referring to the real time bytes/second being transferred? Are you asking for the amount of bytes transferred over time?
The OID's you found tell you how much data has been transferred, i.e. octects = bytes. Using that value you can calculate how much data was transferred between the current polling period, and the previous polling period, giving you an average over time. For higher resolution decrease the polling interval.
You will need to join your Meraki device to SNMP neighborhood.
First of all you need to decide which version of SNMP your are going to use. I normally go with version 2 which uses a community string to authenticate but the more secure would be version 3 which uses a combination of username/password.
Version 2 by default sets the community string to "public" which if I rememeber right is already set on the Meraki once SNMP is activated. As log as you use the same community string on the software you are using to monitor the network along with the Meraki it should display immediately.
Within the monitoring sofware I'm guessing you have a network map. Make sure on this network MAP you assign the correct port to the Meraki or it wont display statistics