When I select Run Configurations from the Run menu I see no tab named Classpath as in previous versions of Eclipse. Where do I set the runtime classpath?
In Eclipse look at Project -> Properties for the project you want a runtime configuration. Choose Run/Debug settings and choose new or edit one you already have. When creating a New run/debug config make sure to choose Java Project. When the dialog box opens you will see a tab for classpath directives.
I stumbled upon this when developing a plugin for Eclipse - apparently the whole classpath business is slightly different in there.
See this FAQ item for more details