If it is not listed, you have not installed it yet. Install the addon, then you'll have a licence box to paste your licence key into.
Have a look at https://confluence.atlassian.com/display/UPM/Universal+Plugin+Manager+Documentation if you're stuck on using the UPM to install and licence addons.
Edit: Sorry, I forgot the other case. If the addon has failed to load/start for some reason, then it won't show in the list of addons. You'll need to read your log file to find out why it's failing in that case.
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