1. Go to the subdirectory ECLIPSE/plugins and create a new, empty subdirectory in "plugins" named "com.umlet.plugin"
2. Download the Umlet3 zip-file and extract it, yielding the Umlet3 directory
3. Copy all files and directories of the Umlet3 directory to "plugins/com.umlet.plugin"; thus, e.g., the file "plugin.xml" is located at "plugins/com.umlet.plugin/plugin.xml"

Now, re-start Eclipse!


To create a new, empty UMLet diagram, 
1. select a directory in the resource navigator,
2. select "File/New/Other.." from the menu or the context menu, and, in the subsequent wizard,
3. select Umlet/Diagram.