You can create users in B2B Data Exchange only if dx.authentication.mode is native, which means that users are maintained in B2B Data Exchange's repository and managed by the system administrator. On the other hand, in ISP authentication, user names and passwords are managed by the administrator using the administration console and stored in the ISP security domain.
By defaullt - dx.authentication.mode is ISP
In this video we shall see the steps involved in changing the dx.authentication.mode to Native and creating the users.