If you are using the political edition of NationBuilder, voter data can be requested from People > More > Import > Voter file import and click the Request voter file button. You'll go to the NationBuilder Election Center where you can finish your request.
Adding voter data will increase the size and cost of your NationBuilder subscription. If you are using another edition of NationBuilder and want to access voter data (permissible use varies by state and NationBuilder does not provide voter data for commercial purposes), you may need the voter add-on enabled. Contact your point person.