I'm basically trying trying to find this setting:
From this question, but can't find it on this Snow Leopard MacBook. My version of that settings window has the slider, plus a list of resolutions, but not the "auto brightness" checkbox.
Have you tried doing an SMC Reset to see if the option appears in the Display preference pane?
Also, some information about what operating system and model you're using would be helpful.
Edit: After further research, it appears that the model in question does not have an Ambient Light Sensor (ALS) which is required for the automatic brightness adjustment feature.