I can't get Java to work - missing something
 

Even though I have downloaded the latest Java for Linux from the Sun website, the Java-needing parts of the online classroom in my graduate courses remain blank. (It's a Blackboard classroom.)
The only solution I've found is awkward and inconvenient. I had to install Windows XP in a virtual machine; download and install the Windows version of Java; and go to the online classroom in [shudder] Internet Explorer (not Firefox; the Java problems are as present in Windows's Firefox as they are in Linux's Firefox). Only in Internet Explorer do my Java applets work. I tried it from another Linux distro (Ubuntu), and found the problem to be better but still there: some of the Java applets work, but not all.

Obviously, something Java-related is missing from my distro, MEPIS 8. But I can't figure out exactly what; I don't know what I'm looking for. All I can do is download random Java-related software and some it makes my broken applets work. I'm all the more puzzled because installing the Java runtime environment (Linux version) doesn't solve the problem, but installing the WIndows version of Java in Windows XP solves the problem there.

Chances are your Linux installation contains the wrong Java engine. Present Linux distributions are shipping with an experimental (read: broken) open-source Java version called "IcedTea". To verify that this is the source of your problem, run Firefox and type this into the address bar:

about:plugins

A list of plugins will appear. Scroll down to the Java plugins. If you see any modules including the word "Icedtea" in the list, disable them.

Go to http://java.com and download the Linux Java runtime engine. Install it. Then follow these instructions to link the right Java browser plugin to Firefox:

http://digitizor.com/2009/01/05/how-...-linux-distro/

And from your post, it seems you are having the same problem in Windows, and for a similar reason, The solution is the same -- get the official Java plugin and make it visible to Firefox.

Okay, trying that. Where should I install Java? Does it matter? I downloaded the file onto my desktop.

by default, it is installed at

/usr/lib/jvm

you can see it through this command line from terminal.

update-java-alternatives -l


and, you might have some jvm installed there as well.