You need to use the default macOS unzipping tool (Archive Utility.app) to unzip the downloaded file.
If you previously downloaded WriteMapper in the form of a ZIP file, unzipping the file downloaded from our website with other unzipping tools such as Unarchiver will lead to this issue occurring.
To confirm this is the issue you're facing, you may expand the details in that error message page by clicking on the "Show Details" button. The message details should include a line (about midway through the details) stating that the "Dyld Error Message" is "Library not loaded".
However, from WriteMapper 2.7 onwards, the downloadable macOS app comes in the form of a DMG installer, which will allow you to install and use the app without needing to unzip it at all.