Good News:
Here's a simple rundown/walkthru of how to open/install a .JAR on your Blackberry:
(First, thanks to Sunkast for letting us know how to "explore" under the media icon, I was going crazy with this phone thinking I couldn't explore what was on the memory card or phone! [I used to have an HP Ipaq 6945 which was a bit more easier to navigate!]... I also just learned how to do this)
1. Download to .JAR to your PC
2. Link your Blackberry to your PC via the USB cable
3. Open "Computer" in the Start Menu
4. Go to the folder/location on your PC where you stored the .JAR file
5. Either copy or cut the file (In case there's any oopses, best to copy)
6. In the Drive/Device list under the "Computer" screen find your Blackberry
7. Paste the file either into the root of Blackberry or a subfolder, either will suffice.
8. Unhook Blackberry from USB cable
9. In the applications on your phone (or on home screen depending on Theme) scroll to the "Media" icon
10. Click the "Media" icon to launch it
11. Press the menu button and select "Explore"
12. Locate the .JAR file either on the device memory or memory card (check both as the Blackberry seems to just sort of stick system memory and card memory together lol)
13. Click the file, and it should prompt you to download (install)
14. Click "Download" and you should be golden.
Now, the bad news:
I was trying to do this for Tibia MMORPG and even though the game installs (already tried the direct-phone install with browser and no dice either) don't think its code is compatible with the Blackberry as it has a startup error and won't load.