1. Quit Slideshow if it is running.

2. Hold the Option key and use the “Go” menu in the Finder to Show Library

3. Delete these files and folders if they are present:

To reset all the app settings:


To reset all the app data:

~/Library/Application support/Apimac/Slideshow/

4. Launch the app in order to recreate from scratch the resources it needs.

See also: 

App can’t be opened because it is from an unidentified developer