There are two methods of doing it. First, I'll describe the easy way:
1. Open a new Finder window. Navigate to the folder you want.
2. Click and drag the folder onto the right portion of the dock.
3. If done right, it should be visible on the dock, by the Trash:

Second, the long way, which lets you put the folder anywhere you want on the dock:
1. Open a new Finder window. Navigate to the folder you want.
2. Click the folder ONCE, or press enter.
3. Add .app to the folder's name. Press enter to confirm it.
4. Drag the folder where you want it on the dock.
5. In the same finder window, remove the .app part.
6. If done right, it should look something like this:



Sign In
Create Account








