Okay, I'm totally and completely confuddled about Java and Mac

My MacPro and MacBook both run Leopard 10.5.8.

There is a client web site where we up- and download large files. It can operate in one of three ways:

1. A browser wherein one can upload five files at a time, but one must search for them in the browser's window and follow the site's protocol. It works, but its cludgy.

2. A Win-only applet

3. A Java applet that is richly featured. Drag and drop. Up-and download of entire folders with a single click. File previews before downloading. Much more. This is clearly the way to go.

But .......

When I log on the site with either Mac, it only offers me the choice of using the browser. It knows I am on a Mac and doesn't offer me the Win app. When I try to use the Java app, it tells me Java is not installed.

I have tried this with both Firefox and Safari, with the same result.

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I thought Java was installed with the OS. It isn't?

I can find NOTHING about Mac and Java on the Google apart from some articles from a few years ago saying Java didn't work so well with Panther.

Can anyone offer any help/advice?