When importing a DXF or shapefile as a background, it does not show up in the model, even when pressing zoom extents.
What you're seeing could be due to some extra elements being located far away from the main drawing. When you click the zoom extents button the model will zoom very far out in order to display everything.
First, confirm that the correct units were used when inserting the background. You can right click on the background layer and choose "edit" to change this.
This could also occur if your model is located in a different place (based on the X/Y coordinates). Try going to View > Zoom > Zoom Center and zooming to a coordinate that you know is within the drawing. You could also bring the background file into a blank drawing and locate the center coordinates of the drawing that way. The coordinates in the standalone version of the software are shown in the location in the screen shot below. Once the coordinates are located you can use the Edit > 'Select All' feature or CTRL + A to select all the elements in your model and drag the model file over to a location of the background.
Also try, saving and reopening the model, or opening the DXF in a program such as AutoCAD or MicroStation to check for problems. When checking for problems with the file in AutoCAD or Microstation check for unsupported entities such as Mtext, Arc's, etc... They will not show up and will need to be converted to a single line text and regular polylines.
Lastly, if your CAD drawing has data in multiple layers, try placing all entities on one layer, then re-save and try again.
Text is not displaying in my DXF background