Hi,
first and foremost, I'd strongly suggest to always use the latest ns-3 release. At the moment it's 3.26.
About your issue, try installing mercurial eclipse from the Eclipse Marketplace (the one you open from within Eclipse, there's a menu item).
If you search for "mercurial" you'll have only two choice - choose the most recent one.
If you have issues in installing it, also check that you're not behind a firewall. I heard that sometimes, in some countries, the Eclipse marketplace is erroneously blocked.
Cheers,
T.