I'm assuming you mean Free tools - PayPal Add to cart button.
to add additional options, you need to hand code the option after the button is created.
options are passed using form elements with a naming convention of:
on<int> - This passes the name of the option (where <int> is a number starting at 0)
os<int> - This passes the value of the option (where <int> is a number starting at 0)
if you set up a size option in the button with small, medium and large, the code genewrted will be:
Size:<input type="hidden" name="on0" value="Size" />
<select name="os0">
<option value="Small">Small</option>
<option value=" Medium"> Medium</option>
<option value=" Large"> Large</option>
</select>
so to add a color option, you would add another hidden element named on1 to pass "Color" as the name of thew option and a select list named "os1" to pass the selected color:
Color:<input type="hidden" name="on1" value="Color" />
<select name="os1">
<option value="Red">Red</option>
<option value="Yellow">Yellow</option>
<option value="Blue">Blue</option>
</select>