You can reach this dialog via the menu entry Analysis \ Symmetric Encryption (classic) \ Ciphertext-Only \ Addition.
This dialog is part of the automatic analysis of the encryption algorithm using byte addition or binary exclusive-OR.
The calculated key length is displayed in this dialog. If the length is considered incorrect, it can be altered. The user is also required to enter the character which is expected to occur most often in the plaintext.
To view the plaintext, click on Decrypt; to terminate the analysis click on Cancel.