In WordPress world, site owners find and install plugins via the wp-admin dashboard - see screenshot below. Searching for discourse pulls up only an unknown (to me) plugin called PrimeTime WordPress + Discourse SSO which then lets you install it directly or click through to learn more in the plugin library. I have no idea about this plugin and how it works or compares to wp-discourse - but I know enough about discourse to steer clear of it.
The steps to get the wp-discourse plugin are different and a little hidden/hard to find. You go to GitHub - discourse/wp-discourse: WordPress plugin that lets you use Discourse as the community engine for a WordPress blog and click on the download zip button in bottom right corner. Then upload the zip via wp-admin dashboard to install.
I know there are probably good reasons for not having it in the WordPress plugin directory - mainly I’m not sure how versioning is handled. But I think we’re setting ourselves up for more support requests by making the plugin that we know works well so hard to find.