It sounds OK:
1) The .jar file in question should be something like "httpcore-4.0.1.jar". Please confirm the name. 2) Adding this to "External Libraries" is absolutely the correct thing to do. 3) For compiling in the Eclipse IDE, you must: a) Select your project b) Select "Build Path" > "Add External Libraries" 4) For running in the Eclipse IDE, you must a) Select "Run" > "Run configurations". Choose your (Java Application?) configuration. b) Select "Classpath" > "Add External Jar" 5) For running outside of the Eclipse IDE, you must make sure the .jar file is present and your -cp (Classpath) argument is set correctly.