Some Open API classes may come from plugins (like the SysML plugin, for example). In this case, make sure that jar files from a plugin directory are added to the compiled classpath. Also, your plugin.xml must define the required plugin too.
To find dependencies
- In the Main toolbar, click Tools > Development Tools > Find Plugin Dependency by Class Name...
The Development Tools menu item is available once the Development Tools Plugin is installed. See the Installing plugins page.