Register button is there but it doesn't show (Membership)

Hi,
I am installing Membership in a website and I have problems with the register buttons.
As you can see in the screenshot the button is not there, instead, there is a small grey line. When you click the small grey line, you go to the next step in the registration process, so I assume that the small grey line is the button as it works like it is.
Why is not looking like a button? Any idea?
Thanks in advance,
Xènia