When you click the Logon button and no systems show up in the Available Systems dialog, like in Figure 1, it can be one of two things. 1) There haven't been any systems configured, or 2) The administrator has not given you access to the configured systems.
1) To make sure there are System Instances setup go to the Administration tab in Data Workbench and select the Manage System Instances button. If there aren't any System Instances in the list click the Add System button, select the system you would like to add, click the Test button to make sure you can connect to the system and then click OK. After you are done with this please follow the next step to give users access to the system.
2) If there are System Instances in the list, or if you just added a System Instance, you need to give users access to the system. To do this select the user or group in the tree that you would like to manage the System Instances for. Figure 2 shows selecting a user. Once you have selected the user click the Permissions tab. At the bottom of the tab the available system instances are shown, click the Allow check box next to the system you wish to give the user access to. Then click the Save button located at the bottom right of the screen. The system instances should show up in the users list the next time they log into the system.