Jar files are for Java
OS x Lion doesn't have Java installed by default as Apple is depreciating it, meaning they don't want to run that anymore on Mac's.
There is a lot of security issues lately with Java, you should seriously consider the source of that jar file because it's likely malware or malware is on your machine if your seeing this in a window or something strange.
You can install Java for Lion here:
If you have Java installed, use Software Update to get it current.
Next use the Java Preferences in your Utilities folder to activate it, in 10.6 and 10.7 Apple disabled it if it's not being used as a security measure due to driveby malware on websties.
A .jar file is a java based 'rar' file. It is basically a container file often used to package up java based applications. They can be run:
Try here for more detail:
Do you want to unarchive the .Jar file or run it?
If you want to run it do this: spotlight>Java Preferences> Drag Java SE 6 to the top of the list. (assuming you have the latest Java) Now, you should be able to run the file, no problem.
If you want to unarchive the .jar file; see what's in it. Download The Unarchiver below. It will open almost any file.
Hope this helped! Let me know!
Don't know if anybody still having problems launching jar files, but I was able to create an app that uses the Terminal command to launch a jar and made a simple app that permits the user to again launch with only a double click. One just needs to right click jar->get info->set Dot-Jat as default for all, and the rest is history. I only been able to test it on my Mac OS X Mountain Lion, but I believe it should work for Lion also. To contact me to get the Dot-Jar(BETA) app, send me an email to email@example.com.