i had the same problem, download the latest drivers from nvidia site worked for me.

Hi, 

Try reinstalling the Nvidia drivers and make sure your system is running the latest Bios version, you can download the drivers and the Bios from the Dell Support site. If the issue continues, send me a Service Tag in a PM so we can look further into this. 

Hi,
I have the exact same problem. I reinstalled the bios, nvidia drivers several times still the brightness is stuck at 100%. The slider and buttons work but brightness does not change. I bought the Alienware 15 R3 less than a month ago. I cannot PM you for some reason.

Hey Starlight, not sure how much of a solution this is. But I found that reverting the nvidia gtx driver back to the last one (2/17) works to restore that function. Kinda sucks but it literally hurts my eyes to look at a white screen in a dark room at 100%. Hope this helps you. 

I tried doing rolling back the driver too, but it didn't work for me. Could you please send me the exact link for the driver you are using or how i can get it? My email is epicraiyan@hotmail.com

Screen brightness contro in the latest Nvidia drivers for the AW15R3 is broken.

Download and install the latest driver released by Dell.

nVIDIA GeForce GTX 1050 1050Ti 1060 1070 1080 Driver Driver Details | Dell US 

I bought my alienware 15 r3 yesterday and it works wonderfully except for not being able to change the brightness. I have updated the BIOS and rolled back 2 drivers and nothing has changed.

I am trying an older driver as we speak. I use my laptop at work, (yeah im the guy with the giant laptop on my desk), and really need it to be tunable. 

Any updates on this?

EDIT: Reverting to a February driver fixed it. 

EDIT AGAIN: it stopped responding to changes again. Its like staring at the sun. 

still not working

You may have an issue that Windows Update is constantly updating the driver to a new version after you installed an older version which make it work again? I had the same problem. Check another thread started by me here about this and how to prevent Windiws Update from updating the Nvidia driver.

Maybe completely uninstall the current Nvidia first (http://www.guru3d.com/files-details/display-driver-uninstaller-download.html) and then do a clean install from DELL's latest released Nvidia driver?