The correct classpath must be specified when compiling your (plugin) code or running *.jar files from <modeling tool installation directory>\lib, as well as all its subdirectories must be included into the classpath.
You can open the properties file in <modeling tool installation directory>\bin and check the CLASSPATH property to be sure what files must be included.
Make sure that following *.jar files are added in such order:
brand.jar (if the file exists). This file is needed if the Cameo Enterprise Architecture or Cameo Business Modeler program is used. MagicDraw does not requires this file.
brand_api.jar (if the file exists)*
Rest *.jar files. Order of them is not important
Make sure that only the *.jar files coming with the current program version you are included. If you have upgraded your modeling tool from an earlier version, the lib directory may contain files from the earlier version. Do not include these obsolete *.jar files.
Keep in mind that some Open API classes may come from plugins (like the SysML plugin for example). In these case, make sure that jar files from a plugin directory are added into the compiled classpath.