How do I field map a custom field in Marketo to Salesforce.com?

To check the fields, you can go to the Admin section -> field management -> Field Mapping
You can check to see the exact Field labels for country, industry, and company. You can see if they are mapped at the lead level or contact level. If the Marketo field is mapped to both, you will both fields in Salesforce when the lead fills out the Marketo form instantly. If the field is mapped only to the Contact level, you will only see the field in the Salesforce once the lead fills out the forms AND gets converted into a Contact. 

Make sure that the field labels are exactly the same as what you inserted into the form. Field names are case-senstive and make sure to use the underscore if they have it. 
Example:
"Company_Name" is NOT the same as "Company Name"

If your fields do not match exactly as they are written in SFDC, you won't see them. 

If they are standard fields such as email address, phone,  etc., they should already be mapped if the sync is correct.

If they are custom fields, they must be created in Salesforce and be visible to Marketo Sync user in order for Marketo to see in the forms.

 

I hope that helps

A field created in Marketo (with very few exceptions - like fields for Sales Insight) CANNOT be mapped into Salesforce.

However, you can create a batch smart campaign to populate a Salesforce created field with that Marketo custom field's value

1.) Create the field in Salesforce and make sure it's visible in Marketo (wait 5-10 minutes for it to sync over)
2.) Create a Smart campaign that says [Marketo Custom Field] is not empty
3.) Flow 'Change Data Value', Attribute = [Custom Salesforce field]  New Value = Token for Marketo Field, i.e. {{lead.customfield}}

Make sure to spot-check the list of leads that will be affected before you activate!