There is two ways to create a zeroLength element in OSN.
1) if you need to add zeroLength elements at the end of frame elements you can use the Add/Delete ZeroLength… menu. This is useful for defining hinges at the ends of frame elements. The local x-axis of such a zeroLength element is parallel to the local x-axis of the frame element.
2) in all other cases you will need to manually create two nodes that are in the same location and then add an element using the Add… menu. For this case you will also need to use the Assign menu to define the local x-axis of the zeroLength element and if your problem is 3D use the element rotation to define the local y-axis (by default the local axes coincide with the global axes)
For both of these approaches you will need to define the zeroLength element properties using the Define menu. P, Vy, Vz are the translational properties along the local x, y, z axes and T, My, Mz are the rotational properties about the local x, y, z axes.
Once you have the properties defined use the Assign menu to assign them to the desired elements in your model.