Importing a 2D DXF / DWG file in menu File - Import... requires an exisiting project with a 2D view. You can't open a 2D DXF instead of a project file in menu File-Open... .
After you have selected a dxf file for import you get the preview dialog first. Here you can choose the imported content and the dimensions of the drawing. Depending on the original drawing software dxf files can be oversized up to 50 km. This might cause some questions in the next step. When you quit the dialog with OK, the dxf file picks with a preview at your mouse cursor and has to be placed in your plan with a left mouse click. If you need one copy only cancel input after that with ESC. With oversized files it happens that you see neither a preview nor an imported symbol because you are in the center of the 50 km drawing. In this case press HOME on your keyboard or choose "Show all" from the upper toolbar.