The TreePress Charts Add On plugin requires the core TreePress WordPress Premium plugin to work.
To add the Charts plugin into your site, visit the TreePress sidebar and locate the Add Ons tab. The plugin can be purchased and installed from there.
Once the purchase is complete you will receive an email with a download link and your licence details. If you need to install the plugin manually, you can use this link to download the software and then install it directly from the Plugins menu.