Arduino will not open

I have tried to download the Ardiuno IDE program multiple times. Once I click on the icon, absolutely noting happens. I have checked tutorials and it doesn't seem like I am doing anything wrong. Any suggestions?

I have a Macbook running version 10.5.8 if that helps anything. Thank you!!!

PowerPC or Intel based Mac???

I think the highest PPC supported was version 0009

I'm having a similar problem under Windows 8. The Arduino.exe program starts up, displays the initial message and then just disappears after a few seconds. Version is the latest stable release, 1.0.5-r2. I got it to run OK using Windows compatibility mode, Windows 7 in this case. PC hardware is an AMD Phenom two-core desktop, Windows 8.0 fully patched, 8GB RAM and plenty of HD space. The system Task Manager says the version of Java I'm using is 6.0.160.1, up-to-date as far as I know.

The Arduino I'm working with is a Nano, plugged into a USB 2.0 port and running a blink (green LED) out of the box from new perfectly well.