Cheers SJS66,
The previsualization of bright green and red lines does not show up on very complex geometry. SU seems to fail at some point.
If you don't double-click anything, just select all the faces you wish to subdivide, and choose
right-click>TwilightV2>Faces>Subdivide it will default to a single subdivision.
Simply hit the number 2 on your keyboard and hit enter - it should give you 2 subdivs. 3 then enter, and so forth. Be careful not to go too high. You can't go over 10, but we recommend you do not exceed 4 in most any case.
If you single-click a selected/highlighted face and move your mouse away from the point you clicked it will increase or decrease the subdivs interactively. perhaps after the single click you are moving your mouse away from the point where you double-clicked? Keep the mouse very close to the point where you initiated the single-click in order to carefully move the mouse curser slightly up or down to increase/decrease subdivs. When you have the look you want you THEN double-click (click a second time) to "set" or "fix" the subdivs to the number they were adjusted to when you were moving the mouse, then hit "enter" key to "commit" the subdivs operation. The word "double-click" is probably confusing there. It's really "click once" to start subdivision adjustment pre-visualization and "click a second time" to stop subdiv adjustment pre-visualization.
If that doesn't work:
Do you have a screenshot of the model you are trying to subdivide?
Or can you share the model via filesharing service? You can PM the link to me.
Keep in mind this is a "flat" subdivision, it's simply subdividing the face, it is not trying to "round" the surfaces by interpolation as the subdivision modifier does in some applications. For that functionality, you would need ThomThom's SubD plugin.
Typically I was half way through making a video of it to send to you when I worked out how to do it. I wish I had a dollar for every time I'd done that... 
