Most often your problem is the workgroup you are in. If you go to the Workgroup list you should see some new workgroups in the list. They will be labeled DXF or IGS Layer. As you click on the workgroups you will see that some or all of the geometry becomes blue, yellow or burgundy. When the happens you are in the workgroup containing that geometry and can work with and machine this geometry.
If you see the workgroups but still see no geometry you may have the geometry turned off. On the top of your GibbsCAM window are a series of buttons. The button on the far left will turn on and off the appearance of geometry.
You may also need to perform a Shrink Wrap. This function can be found in the View menu and will adjust your stock definition so that all the geometry and solids data is fully contained within the defined stock.