[news.eclipse.newcomer] Main-Class manifest is missing?

["Krister Henningsson" <h_krister@xxxxxxxxxxx>]

Hello ng,

After building my first .jar with ant
(build tool core 3.0.0 - ui 3.0.2 - apache ant 1.6.2)
running the class/jar from the ide is posible but launching it from 
doubleclick is not.
I get the following java virtual machine launcher error:

Failed to load Main-Class manifest attribute from
C:\Eclipse\pressMe\testjar.jar

does anyone know what the most likely remedy is?
Kriss