You have a Salesforce lead field that is not currently in Marketo but you want to have in it Marketo for use.
Login to Salesforce as a System Administrator user.
Identify the Salesforce profile that is being used by the Marketo sync user by clicking on Setup -> Manage Users -> Users
Once you have identified the Salesforce profile being used by the Marketo sync user, you will need to make the Salesforce field visible to the Salesforce profile of the Marketo sync user. You do this via "Field-Level Security" setting for the field in Salesforce. In Salesforce, click on Your name at the top right of the screen -> Setup -> App Setup -> Customize -> click the Salesforce object the field is under. i.e. Leads, Contacts, Accounts, Opportunities -> Fields -> click on the Field Label hyperlink -> click on the "Set Field-Level security" button -> check the "Visible" checkbox next to the Salesforce profile that is being used by the Marketo sync user.
Only the "Visible" checkbox must be checked. The "Read Only" checkbox must be checked. In the following screenshot below, notice the different checkbox options for the "Backup User" Salesforce field. However, only 1 option combination will work correctly - i.e. the option with only the "Visible" checkbox checked.
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Some standard Salesforce fields even though you have correctly followed the instructions above, simply do not sync to Marketo. One example of this is the "Rating" field on the Account object.