Impossible to add fields

Hi,
I just buy Gravity Forms.
I install it and activate it.

I created a new form but it's impossible to add fields. When I click on "Single Line Text" or a another field, there is nothing.

I don't understand why.

I try to deactivate all plugins, same problem. I try to change theme, same thing.

Is there a real admin here from Gravity which could see my admin area and tell me if he find a solution ?

I try too to activate the "No-Conflict Mode" in the setting of Gravity. Without effect :/

Thanks in advance,
Regards

Hi tmseyecar,

Thanks for your help. I learn how to identify errors, it's a good thing, thanks.

I see that I have an error when I'm on the page to edit my form :
Erreur : TypeError: jQuery.toJSON is not a function
Fichier Source : http://xxx.com/wp-content/plugins/gravityforms/js/form_editor.js?ver=1.7.5
Ligne : 2348

And each time I click on a field in order to add it, the console of errors give me this error :
Erreur : TypeError: jQuery.toJSON is not a function
Fichier Source : http://xxx/wp-admin/admin.php?page=gf_edit_forms&id=2
Ligne : 3104

Maybe it's could help to identify the bug... :)

Regards