Usually, new SFDC fields become visible in Marketo after the next sync has started. The schema is one of the first items that are synched, before leads, contacts, accounts, etc. are updated. If you are in the Admin - Field Management, you may have to refresh your browser to see the new field.
However, if the field does not show-up, check the field level security settings in SFDC - the user account for Marketo must have the permissions to read and write to that field. Then wait for the next sync.
If the field still doesn't show, you may have a support case that you can log in the Support section of this community site.