Hi,
I'd just simply search for "plugins" and then click this entry:

From here, it takes a few moments (about a minute or two) for the list of all the applications/plugins to populate, but then you can search for Human Resources and activate as you need:

Alternatively, you can login to the ServiceNow Developer portal, click your profile in upper-right and then click "Activate Plugin" and look for HRSD that way as well. I prefer to do it directly in the instance, if available.
If you continue to have issues with your PDI, please use the "Give feedback" button a the bottom of the page on the ServiceNow Developer site.
Please mark reply as Helpful/Correct, if applicable. Thanks!
Please consider marking my reply as Helpful and/or Accept Solution, if applicable. Thanks!