You can go to https://nmp.nirvanix.com/accountStatus.aspx. The account status page has the ability to generate CSV reports or you can view the usage via the flash graphs for each subset of the system that was used.
If you wish to get the usage through an API call you can call GetAccountUsage which will return the usage for a specific account. This can be used on the master account as well but will only return the usage for one application. If you have multiple applications you will need to log in to each one with the master account and aggregate the usage from each app.
Let me know if you have any questions.
Regards,
Barry R.