To access the Source and the SourceRepository, right click on the source in the left pane that lists sources. Perform an "Edit", then look for the field called "Call Number". You can put the file path here if you like.

EDIT: In general you are correct:

REPOsitory is a library or any place that maintains/holds information.

SourceRepository is the way to find the source at that Reository, i.e. Catalog or Call Number

Source is a book, or a document, or whatever

Citation aka SourceCitation. is the page or location of the specific information with in the source.