The museum has no public catalog, aside form a few showcased items on its Collections page. The best you can do is to search large, full text collections for citations to items from the museum, or look at the museum's publications.
It is best to speak with someone in the MU Archives before visiting, to make sure they can pull the material you need in timely fashion. Call 573-882-7567 during its open hours, which are 8AM and 5PM Monday-Friday.
Archival collections are organized by provenance (which office/entity on campus created or used the documents), stored in boxes, and provided with written inventories and/or finding aids, which may be searched or browsed.