At design time the implementations for the classes of a third party library are provided in a design time package. So, if your IDE cannot find the classes then the Teechart design time packages must be missing from the IDE.
I can't say why they are missing, and which ones you need to add to the IDE. From memory I think that you can add them from the Component | Install Packages menu.
Perhaps the best solution would be to uninstall Teechart, and go through the installation process again.