Initially you see the "Bounds and Frame" menu. Here you can set the names of the X and Y axis variables and the X and Y axis scales, and then label the axes by clicking "Draw Frame". Then change to the "Direction Field" menu (click on "Bounds and Frame" and choose "Direction Field"). Enter the differential equation in the box after "dy/dx". Clicking the "Plot field" button will plot the direction field. Enter initial X and Y values in the two boxes after "Initial point [", and click "Plot solution" to plot the solution to the differential equation with those initial values. Now change to the "Isoclines" menu, and you can plot isoclines. For the isocline f(x,y) = c, enter c in the "Slope value" box.
The fourth menu is "Function Plotter". Here you enter a function of x after "y = ", and click "Plot function" to plot this.