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.
macOS 10.13 ScreenSaver changes, issues and bugs
Today's release of macOS 10.13 brings a number of changes that affect screensavers in general, and iScreensaver specifically. If you have comments or questions, please submit them here to the forum.
Please read the tech note:macOS 10.13 High Sierra Technical Note