I unzipped a vanilla Eclipse 3.7.2 and copied uml.jar into /lugins" directory and works as charmed:
Later, following the UMLet instructions:
How to start UMLet as Eclipse-Plugin?
Download the "com.umlet.plugin*.jar" file and copy it to Eclipse's plugin directory.
To create a new, empty UMLet diagram in Eclipse,
1. select a directory in the Eclipse resource navigator;
2. select "File/New/Other.." from the menu or the context menu; in the subsequent wizard,
3. select "UMLet/Diagram".
http://www.umlet.com/faq.htm