At the present time, iScreensaver Designer 5.2 provides support for macOS 10.13 High Sierra, but there are some functional limitations.
Please read the 10.13 High Sierra Tech Note
as well as the Release Notes
Due to changes made by Apple, iScreensaver Designer version 5.2 does not currently support macOS 10.14 Mojave.
Please read our forum post
for the latest updates.
[Solved] Mac OS X users asked to update Quicktime on Sierra
Here is a quote from a user
"I'm running Mac Sierra. When I double-click the screensaver, a window says:
QuickTime plugin is not installed. QuickTime plugin version 7.6.0 is recommended. But Sierra is longer supporting this if I understand correctly. Can you help me?"
Please advise what my response should be.