How to start UMLet as Eclipse-Plugin?
1. Go to the subdirectory ECLIPSE/plugins
2. Download the UMLet zip-file and extract it, yielding to the plugins directory of Eclipse

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.