Hi Steve - for revolve you don't need to actually draw in the rail, instead of picking the rail you instead will pick an axis line (specified by 2 points) which the profile will swing around.

So like Pilou says start by selecting your rectangle.

Then fire up the Construct > Revolve command. Then the next step is to pick the revolve axis. You might kind of think of the revolve as being something like a door hinge swinging around and a door hinge has a pin in it that the hinge pivots around. The revolve axis that you will then pick is like that pin.

One thing that can help you figure out what to do next is to look at the prompt in the upper-right area of the screen which tells you what action MoI is waiting for you to do next. So for with Revolve the next step is: "Pick revovle axis start point" - so there it's telling you that you're going to be picking a point, not selecting any rail object. It would say something like "Select" if it was looking for you to pick another curve object.

So now to specify the revolve axis, you want to click the first point here:



Then for the next step pick the 2nd point of the line vertically above that along the z axis like this:



That will then pivot that profile around that axis line which will produce a result like this:



So you don't even need that circle in there at all, you can just have the rectangle all by itself and then do the revolve and pick those points to generate the result.


Note that you will need to have both "Object Snap" and "Straight Snap" enabled on the bottom toolbar to get the points to snap properly - object snap will let you snap to the origin point and then straight snap is needed to get the z snap line, so make sure those 2 buttons are highlighted in orange if you have previously turned them off at some point:




You also might want to check out the video tutorials here:
http://moi3d.com/2.0/docs/tutorials.htm

Both the "6 legged pod" and the "crown of clubs" videos do a revolve in them at some point so if you watch those videos you should be able to see the sequence involved in live action.

- Michael