You must draw four lines. Not one polyline.
Watch this:
![](http://www.heavenlypictures.hu/ac3dplugins/polyfromlines/help.1.jpg)
The opposite lines must have the same number of vertexes:
![](http://www.heavenlypictures.hu/ac3dplugins/polyfromlines/help.2.jpg)
Then select the four lines. It's all the same what selction mode you use.(Group,vertex...). And click Polyfromlines/Create polygon
![](http://www.heavenlypictures.hu/ac3dplugins/polyfromlines/help.3.jpg)
That's all.