How do I triage errors from the AppStore? Where are the log files? Am getting an error message when trying to reinstall TextWrangler from the App Store. The message given is An unknown error occurred. This is not terribly helpful. Where can I look to get more information about this error? Does the App Store application have a log file?
You have to enable debugging first before logs show up in the console.

To activate the Debug menu, open the OS X Terminal utility and run the following command:
defaults write com.apple.appstore ShowDebugMenu -bool true


Debug Logging options
The App Store has various levels of debug logging that can be used, which simply increase the verbosity of the store's logging functions and output more information as the log level is increased. If you are finding the store to be unresponsive or behaving oddly, then you can increase the log level and try your actions again to hopefully capture a cause for the problem in the App Store's log file. This file can be accessed by opening the Console utility and then opening the section called &quot;~/Library/Logs&quot; in the Files section of the sidebar.
The file should be called &quot;appstore.log.&quot; The standard debugging levels will log the program's functions such as which URLs it is connecting to and what tasks it is running, but in addition if you are seeing visual or behavioral oddities on various product pages you can enable JavaScript logging to see if any JavaScript-based errors are contributing factors in these problems.



http://reviews.cnet.com/8301-13727_7-57538104-263/make-use-of-the-mac-app-store-debug-menu/