Educo, if Java is downloaded then it should recognize the .jar format. Try the following steps for Mac:
Launch the Java Control Panel by clicking the Java icon under System Preferences.
Go to the Update tab in Java Control Panel and click on Update Now button that brings up Installer window.
Click on Install Update.
Click on Install and Relaunch.

Let me know if the client still doesn't work, or if there is no Java control panel in your system preferences.

Wickitywocki, when you right click the Launcher to "Open with", is Java one of the options that appears in the dropdown list?

Thanks for the reply Billy. I switched from Mac, so now I'm on a windows.

It's weird, I downloaded it on my mac and clicked it, and it just started. But on windows, it asks what to open the launcher with, and when i select java, it wont launch. like the window will show up for a split second, then just closes.

I downloaded the version of java that is in the instructions when you're downloading the web client. I know everything is installed, because when I do the verify on java website it says everything is installed/up to date.

i know this is going to be a very simple fix, i am just windows illiterate.

I've helped several players with similar problem, the way I fixed it for them is that I made a batch file that launches the client .
I think it's an issue with the latest Java release installer.

You could try installing older version from here:
http://www.oracle.com/technetwork/java/ … 77648.html

https://www.youtube.com/watch?v=3JNhWrENkN8

This is what I just did to make it work on my Windows 8. This is with up to date Java Version 8 update 136.

Open Notepad.exe. type (without quotes).. "java -jar Launcher.jar"

Save As, and save it in the same folder you have Launcher.jar.

Select All Files in the drop down instead of *.txt and save it as Launcher.bat

many thanks ill try that now!

is there an easier way to do this? I'm pretty computer illiterate.

Uninstall all java with the java clean utility.
RESTART
Install clean version of Java, 32-bit and 64-bit.
Most of the time, Windows will not recognize these files because there is a registry error and reinstalling fresh than restarting, will fix it.
If you're running a 64-Bit version of windows, Java doesn't properly run if you do not install 32-bit and 64-bit versions simultaneously, different programs out there are using 32-bit or 64-bit making it really useful to install both versions, if you do not have the correct bit version installed the program will always not run correctly, I have had more than a couple times where simple programs that loaded before do not load until both versions of Java had been installed.

When trying the command prompt fix as suggested in your youtube video, I received the following message: 'java' is not recognized as an internal or external command, operable program or batch file.

The game has been working with no issues for a couple weeks and the file doesn't want to open now.

Reinstalled Java. File opened with no problems whatsoever. If anyone is having trouble opening Launcher.jar i recommend reinstalling like MayDai suggested.