To insert a checkbox either click on the button of the Form Objects Palette or select from the Form Objects menu off the Insert Menu. You will see something that looks like this:
You can then use the Properties Toolbar to assign it a name, a value and specify whether it is checked or not.
Radio Buttons
To insert a radio button either click on the button of the Form Objects Palette or select from the Form Objects menu off the Insert Menu. You will see something that looks like this:
The properties for the radio button are identical as the checkbox.
So what is the difference between the two? There isn't much difference. Often checkboxes are 'on, off' selections, whereas radio buttons are 'either, or'. This isn't always the case, and Dreamweaver isn't capable of the 'either, or' option.