You can visualize an existing database in Vertabelo using the Reverse Engineering Tool. Reverse Engineering is a simple window app (as well as command-line tool) which connects to your database, reads the table and view information and saves into an XML file. You can then import the XML file into your Vertabelo account and see the physical model of your database.
You may read its short, comprehensive documentation here.
Download links for Reverse Engineering Tool 2.0.7: