You need to select a table from which to start from. Kind of like a central table from which Toad will build the relationships out of your entire ER diagram.

First, try checking which one of your tables is the most adequate to start from.

Second, select how many levels of referential tables do you think should be enough for the whole diagram to display (default value "5" is usually fine).

Third, select your display options. Click "Ok".

That's it.