You have 2 choices…
By default, Lightroom will store Presets in a folder on your System Drive. If your catalogue is NOT a default catalogue, but on a separate hard drive, AND If you start up your catalogue on another computer, the presets will NOT be available!
So, the second choice is to store you presets √ Store presets with “this” catalogue! “This” = to an external Hard drive (location).
Choice 1 : The Default…
Go to Lightroom: Preferences > Preset Tab
Clicking on the Show Lightroom Presets folder opens the above on a Mac
and looks like this in Lightroom…
Choice 2: √ Store Presets with This Catalogue…
√ Check Store presets with This Catalogue
You will notice That to get to the Develop Presets folder I had to open Lightroom Settings folder, and then look for the Develop Prefs folder
In Lightroom the presets look like this…
Most downloadable presets come with installation docs
You know what to do, If all else fails?!!!