Thanks for your replies.
Edward - I was not aware that you could import opportunities on an account since everything in Marketo is done at a contact level. How does that work?
Murtza - The custom fields are not showing up in the UI. They are all accessible by the API.
All I'm trying to do is ensure the Opportunity fields are populating the right information, but if I can't see them in the UI, where can I see them? I'm not the person setting up the API and am not a programmer, I'm in Marketing wanting to make sure my marketing system has the information I expect so our reports run correctly. I hope that helps clarify,