Hi Steve, yeah like the other Steve mentions probably setting a construction plane would be a good way to work on an angled area like that.

When you set the construction plane (the button is under View > CPlane), it allows you to redefine the Top/Front/Right views to some local area of your model so you can then go to the top view and draw in it as you normally do, but you will actually be drawing on the angled plane directly.

Some more information here:
http://moi3d.com/2.0/docs/moi_command_reference5.htm#cplane

It's also possible to just draw directly on a planar surface just by snapping your points on to it too though - for example if you set object snap on and then draw a circle you can place the center of the circle on to a surface and then the circle will be oriented to the surface normal when you draw it. Similarly when you do an array grid if you snap the cell size that you draw onto a planar surface for 2 points, it will use the orientation of that plane for doing the array - this all applies when you are drawing in the 3D view and snapping points on to the surfaces.

So you can just directly draw on surfaces in many situations as well without necessarily setting the cplane actually.

If any of that seems confusing, you can also just draw what you want in the regular world top view and then rotate it afterwards as well.

If you want to model and object in the world coordinate system and then transform it onto a location on an object, you can us the Transform > Orient command for that kind of stuff, see here for some examples on that:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4487.61


So there's a variety of methods you can use, there's not necessarily just one best way you can use which ever of these methods that you like the best.

- Michael