Mariola - when you created your form, you had to chose the field in SFDC that you wanted each field in the Marketo form to populate with the updated data from your lead. For example, in our instance of Marketo and SFDC, "First Name:" (Marketo form) goes to "First Name" (SFDC field on the contact record). If someone's name was Stephen in our SFDC database, and they complete a new form and fill out their name as "Steve", the contact record will be updated with the new data. SFDC will now show the lead's name as Steve and not Stephen. That is of course, unless you override Marketo from updating this particular field in SFDC, which can be done in the admin settings. If you used, for example, the "Company Name" field from SFDC, and renamed it "First Name" in your Marketo form, then when someone enters in "Steve" as their first name, this data will replace what used to be in company name area of the contact record with their first name. So if Steve worked for Company XYZ, now it appears that Steve works for company "Steve." This is the field mapping, and this is where your data can be found. Take a look at the SFDC fields where you assigned the data to be delivered from your Marketo form, and you'll find your info there.
To take a look at the data all in one place, create a smartlist with the filter "Filled Out Form", and then create a new view in the results (or edit the default view) to contain only the SFDC fields you used in your Marketo form.