In order to access a Constituent ID field to use for matching in Chimpegration for NXT, you will need to create the merge field in Mailchimp. You can read about setting up that field (and other recommended fields) on this page of our User Guide.
Once the field is set up, you will be able to see it as an option in Chimpegration Cloud. (shown below).
To populate the Constituent ID field in Mailchimp with the data from Raiser's Edge, you will need to use the Export tool .in Chimpegration (click here to read about Export in Chimpegration or click here to read about the Export tool in Chimpegration Classic).