I'm using Graphiz to visualize a DB. So first download and install Graphivz. The visualization is done in two steps, first SQL-Admin creates a file representing the db-tables in Graphviz' dot-notation and the second step is to create the graph with the graphviz tools.
Open the DB-Info menu by selecting File - DB-Info - This DB
Make sure to have set the table-name-pattern filter according your needs (% shows all).Click create DOT-file button and enter the name of the file.
When the dot file is created, the 'create graphic out of DOT'-window appears. The name of the file is held in the clipboard or select it.
Now fill in the other parameters.
Relations in between to tables can only be drawn auutomtically, if a coresponding FOREIGN KEY constaint exist. But you can easily add them on your own. Just edit the dot-File and add such a line at the end before the closing '}' for each relation to create: tablename:columnname -> tablename:columnname [dir = none];