In addition to typing the domain for an equation by hand, for some types of equations you can also select the domain graphically using the mouse or arrow keys. First select the equation you want to add a domain to in the combobox (or type in a new one). Then choose Set Domain from the Point menu and drag out the region along the x-axis you want to plot. (Using the keyboard, use the arrow keys to move the crosshair to one end of the domain, and then hold down shift while moving to the other end.)
The corresponding textual form of the domain will be added to the equation on the edit line (or will replace the previous domain). You can now graph the new equation, which will be added to the queue (it will not replace any previous entries which differ only by domain).
You can use this feature with Cartesian rectangular equations (functions of x) and ODE's. It is not available for polar or parametric curves, since the domains for these equations do not naturally correspond to horizontal regions on the grid. If the text of the current equation is incomplete and can't be recognized as any particular type of equation, Graphmatica will let you add the domain, assuming that you are typing in a Cartesian equation.