By default the Plugins menu in the Admin area is hidden in nopCommerce 2.4.
There is a setting:
General And Miscellaneous Settings -> Security Settings -> Hide admin menu items based on permissions
that needs to be checked if you want the menu to be shown.
This is a bit misleading as by default the Plugins menu should be shown and when the setting is set then it should be hidden only if the current user does not have sufficient rights about it.
Thanks