Go to this link:

minecraft.net/classic/

When you select your mode, a screen on the browser should pop up and look like it's loading. You immediately hover your mouse over it and hold down LShift+LCommand, there should be something at the top of the screen that says "Applet Loader".

Then you still gotta hold down the keys and drag the new window with the name "Applet Loader" anywhere, that is where you'll be playing Classic.

Then you'll see 4 windows one-by-one saying "It's digital signature has an error, do you still want to run?" Click on the box with "Always trust from this source" (sry, i don't have SPECIFIC stuff to say when I try to remember) then click "Run". Then it should work.

Any questions will be asked!