The step completion box is shown at the bottom of the form area. some forms are longer than others and the step completion may be hidden. You can try one of the two following methods to access the button.
1. Maximize the screen.
2. Use the vertical scroll bar on the far right to scroll down the page. Depending on your browser, you may have an addition vertical scroll bar next to the forms itself.

You should also check to make sure you are using a supported browser. Phone browsers do not generally work with online forms as they are not supported. A full list of supported systems and browsers is located in the following helpdesk article:
http://lisdparenthelp.estreamdesk.com/forums/54395/topics/25270  (copy and paste into your browser if the link does not work)