Be sure to make choices as described below.
You have to make four choices along the top of the screen: the group, the cable class, the strut class, and the stress ratio in the window along the right. After these choices are made, if you have chosen a feasible stress ratio, a picture will appear, and you can rotate the tensegrity to see the three-dimensional effect. (If you click above the top black curve in the stress ratio window, you should get a feasible stress ratio. But there are several situations where there is no feasible selection. Most of these have been designated with an X in the strut selection window.)
Keep an eye on the lower window. It tells you the lengths of the cables and struts for your selection and whether there is a feasable three-dimensional realization. If you have made a feasible selection, you can construct your own tensegrity using wire, string, or rubber bands for cables, and dowel rods, tubes, or bars for the struts.