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.
Select the layout engine to render the DOT-file. Try them out and find the best one for your Database. The Graphviz documentation page has more and detailled info.
TIPP:
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];