There is ability to download GHNS in KSysGuard itself - but it's not present in Discover's 'Application Addons' section.
Please update to KNewStuff3. Discover gets the following error: Config group not found! Check your KNS3 installation.
ksysguard is no longer maintained, in Plasma 6 there is the Plasma system monitor for this task. If your issue still happens with the Plasma 6 replacement, please re-open and we can move this bug to the new product, thanks!