How do I look up a customer in the Authorize.Net CIM?

You log in to you account, and go to the Customer Information Manager.
Here you'll choose "Profile ID" from the dropdown and input the value specified in the BOY_E0_BPEXT table in the U_Identifier field for the given BP..
This should result in a single entry, where you can click the Profile ID link to access the Customer Profile.
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The customer profile page will look similar to this.

Here you have an overview of the Customer Profile, with the Identifier iPayment uses when interacting with Authorize.Net.

You can also see each Payment Profile associated with the customer - there should be one payment profile for each card you have added.​