It should of asked you to press a couple of keys to map generic drivers to the keyboard so it will at least minimally work.
You may have to go to the 3rd party manufacturers web site to download drivers for it. If it's a few years old then there may not be any updated drivers unfortunately.
If it's a Bluetooth keyboard make sure the battery's are ok and it's turned on. (And your Mac has a Bluetooth card installed) Unfortunately not all 3rd party keyboards work on a Mac.
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