LEAKED CODE ALERT!

package com.loudpacks.util;

import java.util.Locale;

public final class OSTools
{
	  public static enum OSType {
	    Windows, MacOS, Linux;
	  };

	  protected static OSType detectedOS;

	  public static OSType getOperatingSystemType() {
	    if (detectedOS == null) {
	      String OS = System.getProperty("os.name", "generic").toLowerCase(Locale.ENGLISH);
	      if ((OS.indexOf("mac") >= 0) || (OS.indexOf("darwin") >= 0)) {
	        detectedOS = OSType.MacOS;
	      } else if (OS.indexOf("win") >= 0) {
	        detectedOS = OSType.Windows;
	      } else
	        detectedOS = OSType.Linux;
	      }
	    return detectedOS;
	  }

}

public static void launchClient(String path, String script, String clientUser, String clientPass, String botUser, String botPass, String world, String proxy, String params) {
		ProcessBuilder linuxBuilder = new ProcessBuilder("/bin/bash", "-c",
	            "java -jar " + path + " -Xmx750m -allow norandoms" + proxy + " -login " + clientUser + ":" + clientPass + " -bot " + botUser + ":" + botPass + ":1234" + " -world " + world + " -script " + script + ":" + params);

		ProcessBuilder windowsBuilder = new ProcessBuilder("cmd.exe", "/c",
				 "java -jar " + path + " -Xmx750m -allow norandoms" + proxy + " -login " + clientUser + ":" + clientPass + " -bot " + botUser + ":" + botPass + ":1234" + " -world " + world + " -script " + script + ":" + params);

		ProcessBuilder macBuilder = new ProcessBuilder("osascript", "-e",
	            "tell application \"Terminal\" to do script \"java -jar " + path + " -Xmx750m -allow norandoms" + proxy + " -login " + clientUser + ":" + clientPass + " -bot " + botUser + ":" + botPass + ":1234" + " -world " + world + " -script " + script + ":" + params + "\"");

		try {
			switch(OSTools.getOperatingSystemType()){
				case Windows:
					Process p1 = windowsBuilder.start();
					break;
				case MacOS:
					Process p2 = macBuilder.start();

					break;
				case Linux:
					Process p3 = linuxBuilder.start();
					break;
			}
		} catch (IOException e1) {
			e1.printStackTrace();
			System.out.println(e1.getMessage());
		}

	}

Your proxy string will have to include " -proxy ip:port", I did it like that so it can be optional.