Depending on how NetIDMgr was installed, it might already be running in the system notification area or it might need to be started manually. See the topic starting NetIDMgr for details. Essentially, to open the NetIDMgr window, you either have to click the application icon in the system notification area, select the icon from the start menu, or type 'netidmgr' at a command prompt.
Once you open the NetIDMgr window, you will be presented with a view of your existing credentials, or a message notifying you that you don't have any. The credentials view should be fairly self explanatory, but if there's any doubt, more information can be found here.
Brief overviews of how to perform common tasks are linked below:
A more comprehensive list of how-to topics can be found in the How do I... section.