As of right now the Horizontal scroll cannot be figured to adjust the lightroom or for that matter any Adobe adjustments, which is the main reason because of which, just like you, even I purchased this mouse.
After several google searches and playing around with the logitech mouse options software I was able to find a temp. solution till logitech updates their software.
1) Download, install and Launch the Logitech Options software for MX master.
2) Go to the configuration settings of the Up/Down buttons which are present just next to the Horizontal scroll.
3) Configure these buttons to Keystroke Assignment.
4) Take your cursor to the section where it says 'Enter Keystroke' and press the Up/Down arrow key on your keyboard.
5) Do the same for both the buttons.
6) Now you can adjust your lightroom adjustments by hovering over the adjustment bar and then pressing the configured up/down buttons.
7) If you hover & click then It will adjust the values by -5 or +5
8) If you want to adjust the values by -1 or +1 then click on the actual value till it gets selected and then press the up/down buttons.
Hope this helps.
I will also write to logitech to add Keystroke Assignment option to the horizontal scroll.