Note: Why is search in Activity Monitor disabled? is a similar but different issue that doesn't address the one here (I tried switching views to and from Hierarchically.
The command ⌘-F shortcut doesn't seem to work in Activity Monitor. Also, the Find menu options are greyed-out:
Note: command ⌘-option ⌥-F sets focus into the search bar
Although a search can still be preformed manually by clicking into the search box, the Find menu items remain disabled, except for Jump to Selection, which still doesn't seem to do anything.