How is the national electoral register created?

Democratic Republic of the Congo's Constitution of 2005 with Amendments through 2011

Article 211

The Commission Electorale Indépendante (CEI, Independent Electoral Commission) is established, it will derive its existence and mandate from the post-transitional Constitution of 2005, Article 211. This lays out the functions of the CEI as the organization of the electoral process, including voter registration, the maintenance of voters' rolls, voting operations, vote counting, the announcement of provisional results and the conduct of referendums. It further prescribes that the organisation and functioning be determined by an organic law; such a law has not yet been passed by the legislature.
 

CEPPS Support to DRC Elections Assessment Report, July 18 2016

The CENI, meanwhile, embarked on a full revision of DRC’s voter roll in February 2016 without broad-based support from electoral stakeholders. A full revision of the voter roll will delay presidential elections well past the constitutionally mandated deadline of November 2016, and the end of President Kabila’s term in office in December 2016.

The CEI is responsible for the organization of the electoral process, including voter registration, the maintenance of voters' rolls, voting operations, vote counting, the announcement of provisional results and the conduct of referendums. The decision to embark on a full revision of the voter roll came with widespread public distrust of the current voter roll, which disenfranchises seven million youth and 1.6 million internally displaced people. This is the main official reason provided by the Election Commission for the delay of the Presidential elections.