Presets are stored in a separate folder on each computer. You can Copy them from one computer to another. You can also Store the Preset with the catalog but I'm not sure they get exported when you Export As a Catalog. So I suggest you use the LR Presets folder to store them and just Copy them to the other computer.
Use the button to find the folder where the presets are stored, Copy them to a USB thumb drive, go to the other computer and do the same and then copy from the thumb drive to that folder on the other computer.
Thanks that's really helpful. Quick follow-ups:
1. Where are other settings (e.g. what shade of gray the background for images is, etc?!) stored? Is there a similar folder for them, whose contents can be copied over?
2. In the case of pre-sets, copying the folder is enough, yes? Copying an app from one computer's HD to another woudn't mean both were up and running with the app, but with pre-sets, I take it, it's different. -- is that right?
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1) That is an internal setting, option, AFAIK. If you are getting a different shade of Grey Background on the different computer that could be a monitor setting or a bad profile for the monitor or just the differences between the monitors.
2) I and you, in your original post, said nothing about Apps, like Add-on, Plugin, programs. You asked about Presets.
Copy the Presets out of the Preset folders (there may be more than one folder that holds presets for different LR modules and import/export dialog windows) to the Same folder or folders on the other computer.
As for plugins, like the NIK Collection and others, you will need to install those on the other computer. They don't really Copy over very well if at all.