There are three main options to point your LinkedData sources to the platform:
By providing Data Sample IRI (example RDF a chunk of data from a whole dataset), Named Grap IRIs
By providing a link containing the RDF file you would like to visualize.
By uploading your local RDF file that you would like to visualize.
How to provide LinkedData sources from IRIs?
How to LinkedData sources from RDF files?
Uploading from URL links
Uploading local RDF files
The process for uploading the local RDF files into data preparation workflow is identical to what is presenting the the URL link guide, simple drag & drop your file into corresponding field in From File with RDF tab.