We don’t really know what causes this error… We suspect it might have to do with over-aggressive corporate proxies, anti-virus software, or just an old version of Adobe Air that's already installed on your machine.
Here are some steps that might fix the problem:
- Download and install Adobe Air from http://get.adobe.com/air/
- Once you are done, navigate to http://balsamiq.com/download/
- On the lower half of the page is a Cross-Platform installer. Download the file MockupsForDesktop.air
- Open the downloaded file with Adobe Air. You will get an installation wizard. Follow all instructions and Balsamiq for Desktop should start at the end of this process.