In Citra, go to File > Install CIA and select the decrypted DLC file included in the folder.
The file refers to the decrypted Nintendo 3DS ROM for Senran Kagura 2: Deep Crimson (USA region) including its Downloadable Content (DLC), specifically prepared for use with the Citra Emulator . SK2DC-USA-(DLC)-Decrypted-Ziperto.rar
Contains the base game ROM and the DLC files, both pre-decrypted so they do not require external AES keys to run on an emulator. In Citra, go to File > Install CIA