When attempting to install the Juniper Netscreen Extractions app (http://splunk-base.splunk.com/apps/24436/juniper-netscreen-extractions) I get "An error occurred while installing the app: 500 - [HTTP 500] file could not be opened successfully" when installing from withing splunk and "There was an error processing the upload." When trying to install from an uploaded file. Version 4.2.1 of Splunk.
I unzipped the file once, so to netscreen.tar (its a zip in a zip...) and then it did install but when I went to data input and tried to change the data type from syslog to netscreen it did not exist. (I expected the netscreen app to add a "netscreen" type in this selection box no?)
If you have a directory under "apps" now called "netscreen", with the "default", "local", etc directories under that, then as far as I'm aware it SHOULD be installed correctly. Have you restarted Splunk since the install?
Odd, that file on the site should be fine unless something's gotten modified on the back-end. I can take a look at it tomorrow and see if there's something that needs to be changed for some reason for the recent Splunk versions.