when creating a java project i have found that the best way to implement contents of a .jar is to add the .jar as an external library to the projects build path. To do that, in eclipse, right click on the project and select build path and then external libraries. Follow the instructions to add the .jar. You should then notice it listed as part of your project in the package explorer view.
Let me know if you have questions or need clarification.
I dont have any experience with the BIRT Report Designer. I know that you can accomplish it the way that i suggested, there seems to be a lot of information on the eclipse site regarding the BIRT Report Designer: http://www.eclipse.org/birt/phoenix/