Hi there. No need to apologize for newbie questions.
First of all, where are you seeing "lead info" as a field? There is a tab called "Lead Info" on the Lead record in Marketo, but it's the name of a section rather than a specific field.
Second of all, what type of list are you trying to build? You are correct that your Lead Sources will be blank unless they have previously been populated in SFDC. But there may be another way to get at what you are looking for.