Jean-Marie,
Sorry that you ran into this issue.
Can you please answer some questions for me ?
1) Can you copy paste your installation details here from MyEclipse > Installation Summary > Installation Details ?
2) How are you trying to install the Metrics Plug-in ? Is it through the update site? If yes then share the update site link with me.
3) Can you verify if the version of Metrics plugin you are installing is compatible with the base Eclipse version of MyEclipse ? Ex: If you are working with MyEclipse 8.6 then you have to install Metrics version 1.3.8 which is compatible with base Eclipse 3.5. The update site you can use for ME 8.6 is http://metrics2.sourceforge.net/update/
Let me know how this works for you.