For Eclipse Mars (I've just verified that) you to do this (assuming that C:\eclipseMarsEE is root folder of your Eclipse):
- Add plugins folder to C:\eclipseMarsEE\dropins so that it looks like:
C:\eclipseMarsEE\dropins\plugins
- Then add plugin you want to install into that folder:
C:\eclipseMarsEE\dropins\plugins\someplugin.jar
- Start Eclipse with clean option.
- If you are using shortcut on desktop then just right click on Eclipse
icon > Properties and in Target
field add: -clean like this: C:\eclipseMarsEE\eclipse.exe -clean

- Start Eclipse and verify that your plugin works.
- Remove -clean option from Target field.