I have a problem with Pro Sites (Express checkout)

I'm Sébastien B., i'm french an i'm sorry for my english.

I'm became a premium member on Wpmu plateform and i downloaded "Pro-sites" for my website but, i encountered a problem with the Paypal express checkout button on the selection level page for subscription (front office : mywebsite.net/pro-site/ renamed to mywebsite.net/ma-formule/).

When i press the paypal checkout button : nothing. I stay on the same page without going to the Paypal payment page.
Of course, the parameter is going to the url ( mywebsite/ma-formule/?bid=1) but nothing else : the same page is updated.

What can i do for make it good ?

Thank you for your help.