I've searched for an answer, and - though similar things have showed up - none of them involve the problem I'm running into on Mac. So:

After following the tutorial instructions, I double-clicked on the minecraft-osx.sh file. This is the result I got (after the first instance, I triple-checked that the minecraft.jar file was present, and it was):

"[computer name, username]/desktop/World of Minecraft/minecraft-osx.sh ; exit;
Exception in thread "main" java.lang.NoClassDefFoundError: com/mojang/minecraft/MinecraftApplet
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClassCond(ClassLoader.java:632)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:616)
    at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141)
    at java.net.URLClassLoader.defineClass(URLClassLoader.java:283)
    at java.net.URLClassLoader.access$000(URLClassLoader.java:58)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:197)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:248)
Caused by: java.lang.ClassNotFoundException: com.mojang.minecraft.MinecraftApplet
    at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:248)
    ... 12 more
logout

[Process completed]"

...can anyone help with this?