You need to create the field and values in the Field Management area in the Admin section. Once that's created, you can click the "+" on your form and search the field name to add to your form.
You may need to look at SFDC to add that value to the picklist though.
A
Anonymous
February 10, 2015
How do I create that field in Marketo's Field Management? And how do I ensure that it shows up on salesforce? (i already have the marketo/sfdc sync working)