Hi Grant,
You have to sync the Custom Object over to Marketo.
Here's the general article on syncing custom objects: http://community.marketo.com/MarketoArticle?id=kA050000000KyrHCAS
Here's one about triggers and filters on custom objects: http://community.marketo.com/MarketoArticle?id=kA050000000KyrG
Hope this helps!
Regards,
Sheila