A powerplane layer is generally plotted 'inverted', so that the plot only contains the areas that are not going to be copper when the board is produced. The resulting plot is then 'reversed' to create the large copper areas on the plane. Without doing this, the plot files would tend to be huge (and thus expensive to plot).
Easy-PC contains a feature that allows you to preview the actual appearance of your powerplane before it is plotted. This can be useful in helping you to check that you have everything set up correctly.
Once the powerplane settings have been entered on the [Layers] dialog (Bias set to 'Powerplane', and the correct Net assigned), open the [View] menu, select [Powerplane], then [Show]. If you only have one powerplane layer in your design, the display will update automatically. Otherwise, a small dialog is displayed that allows you to choose which of your powerplane layers to view.
The picture is updated to show the actual powerplane appearance, "underneath" all the other design items. The background is first filled, then all the "not copper" items are "punched through" the filled area to give the final plane appearance.
The powerplane shapes are generated by exactly the same code as is used in the Plotting options to generate the actual plot files, so what you see on the screen should be exactly what you will get on your plot.
For reasons of performance, the powerplane is not re-generated all the time. When you modify other design items, you may see holes appearing in the drawn powerplane, or it might look like the correct spacings around items have not been included. This is perfectly normal. To refresh your powerplane image, simply return to the [View] menu, select [Powerplane], then [Regenerate]. The picture will then update.