Use the plug-in to create another leJOS NXT Project. You will see, that there is an entry called "LeJOS NXT Runtime". It contains the reference to the classes.jar. If you deleted that entry from the build path of your project, it can be recreated by editing the build path of your project. Right click on the project, chose BuildPath>ConfigureBuildPath in the menu. In the Libraries tab, click the AddLibrary Button. Then chose leJOS Library Container, click next, then select NXT Runtime, and click Finish.