1. Do you not see the plugin menu?
Adobe’s way of displaying the menu is misleading.
1.1 Are you looking in the right place?
In Library, it's under Library > Plugin Extras. Lots of people look under the File > Plugin Extras menu.
1.2 Does the menu look greyed-out?
You’ve been misled by Adobe’s way of displaying the menu. They grey out the plugin name, but the real menu is the line below.
1.3 Is there an error message in Plugin Manager?
Often on Windows there is a message about "Malfunctioning" that refers to a file called Info.lua. This indicates that you haven't installed the plugin properly, so remove it, reinstall and be very careful to point to the lrplugin folder, not to a file inside it.
Don't install it in a Downloads folder.
2. EXIF info doesn't change in LR
2.1 Have you remembered to do Metadata > Read Metadata after running the plugin?
When the plugin appears to be functioning but nothing appears to be happening in the Metadata panel, the first question to ask is if you have remembered Metadata > Read Metadata?
In recent versions there has been a message to prompt you, but people sometimes stop noticing it.
2.2 Are you processing raw files?
By default, the plugin doesn't write directly to raw files - it follows Adobe's practice by saving toxmp files for proprietary raw files. But this means that LR's Read Metadata would ignore the sidecars because it expects EXIF to be in the raw files, not sidecars.
Then if you're changing the camera maker or other fields, there's a second check box to confirm you really want to do this.
3. When you examine the files in another app such as Adobe Bridge, is metadata not changing?
3.1 Is the plugin installed in a "special" folder which causes the operating system to stop Exiftool from running?
Have you installed the plugin in a folder regarded as "special" by the operating system? While the plugin dialog box may display normally, the OS could prevent the plugin's embedded Exiftool component from running.
Some people install the plugin in a Modules folder inside the Lightroom program file. This can cause the above problems and I prefer to install plugins somewhere less-special and completely-separate from Lightroom folders, for example in a subfolder in Documents or on Dropbox. Apart from avoiding such permission problems, Dropbox makes the plugin and updates available on more than one computer.
3.2 Did the browser unzip the download? Mainly Mac.
If Safari unzips the download, it looks like it can sandbox the plugin's Exiftools component. So you could just remove the existing plugin and download it again, but carefully saving the zip file to Downloads and not letting Safari unzip it. Then unzip the file with Finder.
3.3 Test the command file
Especially on Mac, the problem may be that the plugin's built-in Exiftool component isn't being allowed to run. I've recently added a Test Exiftool button which should give a basic indication of this issue, or indicate that the plugin is working as intended.
To investigate further, on Mac you copy the command line and paste them into the Mac's Terminal window. This makes the Mac run exactly the same commands as the plugin and produces messages which reveal any problems.
On PC and Mac, the command line can be saved as a text file and used to call Exiftool directly. You'll be running exactly the same commands as the plugin, but in Exiftool's interactive mode you can see messages about its activity and any problems it encountered
In Windows, I think the easiest way is to change the log file's extension to .bat, and it will then run as a batch file.
On Mac I prefer to cut and paste into a Terminal window.