Installation of the Mercurial plugin (1.57) was uneventful and seemingly successful, it shows up in the list of installed plugins. Even after Jenkins was restarted manually, there was no place in the Configure System page where a Mercurial section was displayed. Git and Subversion plugins had been installed and sections for them were present in the Configure system page.
We have installed the plugin through Available tab, and later manually through Advanced tab, after downloading the relevant hpi file. In both instances Mercurial section was missing from the configuration page.