You may have tried to install UML Designer by clicking the Install button
To install this solution to your Eclipse installation, ensure that:
Eclipse is launched and that you click and hold the mouse button while dragging the Install button icon over the Eclipse workspace and then drop it, by releasing the mouse button, onto an open Workspace
You have the Eclipse Marketplace Client (mpc). This will appear in the Help menu as 'Eclipse Marketplace...', and is included with all of the packages available from the Eclipse download page (except Classic Package).
To try to install the UML Designer solution again, you can use this button:
Eclipse Marketplace Client (MPC) is a rich client interface for browsing and installing the Eclipse based solutions listed on the Eclipse Marketplace portal. It is a new feature that allows Eclipse users to discover and install Eclipse solutions directly into their Eclipse installation.