Hi Jen,
As Kenny suggested, by adding Marketo’s ID in your custom view will now show SFDC ID when exporting but take note: The SFDC ID that gets exported will be the 18 digit ID and not the 15 digit ID if you viewed that lead/contact in SFDC.
e.g. when your viewing leads/contacts in SFDC there is a direct URL link to every object. You generally see something like : https://salesforce.com/
0035000001X4XXX - 15 Digit ID
When you export from Marketo, it will be 18 digit ID version. This is something to be aware of in case you do get your other fields working (SFDC Lead ID and SFDC Contact ID) as they will be 15 digit IDs
This still works if you copy and paste that 18 digit ID into SFDC URL when you want to view the lead/contact.
The difference between 18 and 15 digit is that 18 digit ID ios used for SOAP API or backend data base access vs. viewing as a SFDC user in a browser.
Hope all that makes sense.