Also see 
dome-2.skp (4.8 MB)
Depends on the desired result, but to create a dome you should start learning and using the basic tools
then go to plugins
Cool, I will - I am!
on the subject of basic drawing - I am making this shape by drawing - OK I used curviloft for the rounded outsides - but why can I not get the inside wall on the left to close? I took more time over messing about with that than drawing the whole shape! 
ATS Bodywork.skp (301.2 KB)
Because it’s not flat, neither is the right side, looks like you used curviloft for that one too.
If a diagonal closes the face then it tells you the edges are not co planner. A triangle is always flat, a rectangle isn’t.
By the way, if you have new questions put them in a new thread rather than burying them in an old thread. I had stopped reading this one way back and only happened to open it today.
New question new thread.
fair point, sorry! thanks I worked out the issue as you said; I’d redrawn the lines straight, or so I had thought; found the curve and fixed it