gabel8,
MyEclipse 10.x is quite an old release and is no longer supported. The MyEclipse Configuration Center is also discontinued and so you might see issues trying to install plugins via the Configuration Center. You can try the alternate ways of installing plugins either using the downloaded archived update site or using the dropins method.
To install Mercurial plugin, you can download the Merc.tgz file attached in this thread : https://groups.google.com/forum/#!topic/ns-3-users/vfQF2H9wD40
Close MyEclipse. Extract the downloaded Merc.tgz and copy the plugins and features folders to the dropins folder of the MyEclipse 10.x installation directory. If you already have plugins and features folders created in the dropins folder, then just copy the jar from the plugins folder and the folder (mercurialeclipse_2.1.0.201304290948) from the features folder. Restart MyEclipse 10.x and you will see a prompt about the plugin being installed successfully.
We strongly recommend you upgrade to MyEclipse 2017, the latest version based on Eclipse Neon to take advantage of several enhancements and fixes made to the product over the years. Your current license is valid for latest versions as well.
–Swapna
MyEclipse Support