You can use the behavior manager if you don't have access to the behavior code directly:
MEngine *engine = MEngine::getInstance();
MBehaviorManager * bManager = engine->getBehaviorManager();
MBehaviorCreator * bCreator = bManager->getBehaviorByName("LookAt");
MBehavior *behavior = bCreator->getNewBehavior(object);
object->addBehavior(behavior);
MVariable var0 = behavior->getVariable(0); // var 0 is "target"
MString *targetName = (MString *)var0.getPointer();
*targetName = "objetNameToLookAt";If you have access to the behavior class you can do this directly:
MBLookAt *behavior = MBLookAt::getNew(object);
object->addBehavior(behavior);