To install a component and use it in your diagrams, you first need to place the component file inside Circuit Diagram's components folder.
You can open this by clicking Tools->Components.
Then click on the Components folder link at the bottom of the dialog.
This will open the folder into which you need to place the component file. Drag or paste the component file into this folder.
You now need to '''restart Circuit Diagram''' for the component to be loaded.
Add to Toolbox
You now need to add your component to the toolbox so that you can use it in a diagram. Click Tools->Toolbox.
Click on your component on the right of the dialog, select the category on the left you want to add it to, and click << Add.
Click OK - your component is now available in the toolbox and ready to use.