To install a component and use it in your diagrams, place the file inside the "components" sub-folder (create it if it doesn't already exist) within the "Circuit Diagram" folder in the "AppData" section on your computer.
On Windows 7 and Windows Vista, place the component inside the folder:
C:\Users\USERNAME\AppData\Roaming\Circuit Diagram\components\
On Windows XP, place the component inside the folder:
C:\Documents and Settings\USERNAME\Application Data\Circuit Diagram\components\
You now need to add the component to the toolbox. Open Circuit Diagram and go to Tools->Toolbox. Your component should be available in the list on the right - you can now add it to the toolbox and start using it in your diagrams.