If you have Reflection Desktop installed, you can enable it from the IDE options as the TN3270 emulator to launch from within the IDE in order to run or debug your applications.
When you start debugging, the IDE launches Reflection Desktop, if it is not already running, connects it to the enterprise server region, and starts the application in a 3270 terminal. If configured, an existing session profile can be used; see Choosing a TN3270 Mainframe Display.
For more information, see the Reflection Desktop product Help.