Since version 1.7 the Web Viewer runs locally on all major browser (tested in Chrome, Firefox, Safari, IE). Please update to 1.7. In case you are not able to update:
The Antetype web viewer works by default using the local file system to pass the files to a web browser - usually Safari.
However recent upgrades to Safari and Chrome mean that the web viewer does not always work from the file system.
There are a few ways to solve this problem:
1. In Safari enable local files: From the Preferences Panel in Safari Choose the Advanced Tab, then make sure there is a tick in the Show Develop menu in menu bar.
From the Develop menu tick "Disable Local File Restriction"
2. Run a local simple web server in the web viewer folder: You can easily start a simple web server from the OS X Terminal prompt using the following command:
python -m SimpleHTTPServer
There is an OS X Daily Article with more information.
3. Use web sharing and the Sites folder to run a local web server: Prior to Mountain Lion this could be enabled in the System Preferences -> Sharing - Web Sharing.
For mountain lion users who do not have the Web Sharing option please read this article on CNet
4. Use Firefox - Firefox currently doesn't have this restriction in place.
5. Host the files on your own web server: Accessing the files from any web server will get around this restriction in Chrome and Safari.