Do NOT put LUTs in the program's internal LUT folders ... that's not only not recommended, it will lead to really screwy behavior.
There are the two user-added LUT storage locations ... they do publish that in the help documentation. Not that it's a fantastic help file, this is there.
The Program{package} files/Adobe/Common/LUTs folder is the easiest for me. With subfolders I created as above.
After adding any LUTs to the subfolders, you have to relaunch Premiere, or any other of the three apps Ae, Me, or Pr. They only scan those folders for contents on program launch.
And if they are not seeing them on re-launch, it's normally 1) the LUT type isn't usable, like many Resolve created LUTs; 2) there's a name issue or some other file header issue; 3) nearly all other problems are a folder permissions issue between the OS and Premiere.
Some LUTs can be made usable if you know how to open them in a text editor, and simply remove some extra unneeded verbiage in the file header. There's one line typically added by Resolve that gunks Premiere, but I can't remember which it is at the moment.