Hi Itai, first you want to add a new field to the form by clicking the giant plus sign in the corner.  Then, specify which field you want to update.
This is how you do it:
- 
		Click on your new field to open up the field Properties.
- 
		Fill out 'Label'.
- 
		Find 'Field Type'.  If your field type in Marketo is Select, it should automatically show up as "Select".  Otherwise, change 'Field Type' to Select.
- 
		Find the 'Values' field below. You should see # of values next to the Edit option.  If you want to review them, click Edit
- 
		From here, you can:
		- 
				Select a default value
- 
				Add new values
- 
				Customize the Display Value vs. the Stored Value
 
Hope this helps!