The administrator who you are attempting to become must first login successfully on the Applicants and Administrator tab once so that the system can assign the appropriate privileges and "activate" the account. You will not be able to "Become User" (and you may not see them in the Administrators Users grid) until after one successful login.

*Note: If your system utilizes CUUI (Create User Upon Import), this setting immediately authorizes users. If your system utilizes CUUI and you encounter this issue, please contact Support.