The canvas is the white area (that may optionally have a grid) in each of FlowframTk's child windows on which you create your picture. It shares a child window with the preamble editor pane. The canvas and preamble areas are separated by a divider that can be adjusted to allow one side to take up more room in the window. This can be done by dragging the divider. With some look and feels, the divider has small triangular buttons which you can click on to collapse or expand one side, but not all have this function nor can some of them completely hide the preamble pane (such as the GTK+ Look and Feel). You can also use the TeX/LaTeX->Preamble Editor menu item to show the preamble pane.
In select mode (but not when you're editing or distorting a shape) you can drag and drop image files or text onto the canvas to add to the current image. In text or maths mode you can only drag and drop text onto the canvas. If you attempt to drop a file instead, you will get a new text area containing the file's path (or URI).