Bank account setup is in the control panel under Settings > Bank accounts. The setup is most simple with a Democracy Engine account, and you'll be able to pick whether your nation is a nonprofit with tax deductible status and even set a custom disclosure form that someone must click through before making a donation (important for jurisdictions with unique campaign finance rules). 

This video from NationBuilder Live walks you through the process and discusses available payment processors.

You can also watch "Setting up a bank account" in high definition on the NationBuilder YouTube channel.