no text box for user to change price - help!

I have followed your Steps to Setup "Donations as a Product" guide exactly.
Now , when I go to the donate product page, there is an option to add to cart, which is fine, and there is No quantity once i have added it to the cart, which is great. But there is nowhere for the customer to change the price?!
this should appear on the node page right?!
what am I missing? there should be a textbox somewhere, but there isn't.
Please help!
many thanks