I have created an Android project and added an external JAR (hessian-4.0.1.jar) to my project. I then added the JAR to the build path and checked it off in Order and Export.
create a folder (like lib) inside your project, copy your jar to that folder.
now go to configure build path from right click on project, there in build path select
'add jar'browse to the folder you created and pick the jar.