Using RXTX In Eclipse
This is how I add and use RXTX in Eclipse for Win32 Projects, there are probably other ways but it works for me. 
- Copy RXTXcomm.jar, rxtxSerial.dll and rxtxParallel.dll files to the lib directory of your project
- Under Project | Properties | Java Build Path | Libraries
- click Add JARs... Button
- Select the RXTXComm.jar from lib directory
- Jar should now be in the Build Path
- expand the RXTXComm.jar entry in the list and select "Native Library Location"
- Select the project lib directory and apply
Be careful when using System.in.read() and rxtx in win32; It can trip across a known JRE deadlock bug
The above setup didn't quite work for me, so here is an alternative.
- Copy RXTXcomm.jar to the lib directory of your project
- Navigate your package explorer to the lib folder, right click on RXTXcomm.jar | Build Path | Add to built path
- Copy rxtxSerial.dll and rxtxParallel.dll files to the root directory of your project
- Under Run | Run configurations | Classpath tab | User entries | Advanced | Add folder, choose the root folder of your project
- This should be enough just to run it under Eclipse, when deploying a runnable jar, just make sure the dlls are on the same folder as the jar (JVM assumes it for classpath)