OK - MSI has been installed properly, all of its components are deployed, and I reviewed the rights of the Marketo user - all fields are visible to it, no problems there.
Still getting sync errors, though this one is slightly different (see below). Anybody know what is going on?
INVALID_FIELD_FOR_INSERT_UPDATE: Lead: bad field names on insert/update call: mkto_si__Last_Interesting_Moment_Date__c, mkto_si__Last_Interesting_Moment_Desc__c, mkto_si__Last_Interesting_Moment_Type__c, mkto_si__Last_Interesting_Moment_Source__c\n
Did you get to the bottom of this? I have a similar issue where form is passing through new leads but get SFDC error when try to pass through form fill details of an existing lead/person account.