In UE4.9.2, the Plugins window is found under the Edit menu - rather than in the Window menu, as is currently documented here:
Howdy cameronpetty,
Thank you for submitting this error with our Plugins page. I have gone in and corrected the location of the Plugins menu as it now lives in the Edit dropdown. This fix will show live after the next publishing.
Let me know if you run into any additional errors so we can get them fixed.
Thanks again and have a great day!
-Sean G