This is probably due to the fact that the form you are trying to select has 0 leads.
A form is not selectable as long has it has 0 leads. Unfortunately if there is no data in the form, we don't receive the information about the fields that are in the form and are unable to set up the sync properly.
An easy workaround for this would be to subscribe yourself to the list, and then it will become selectable in the AdEspresso Data Sync tool.