It is possibly there but it may be the same colour as the background and will be in Top view, try selecting an ISO view and also try using the "Select All" facility, if nothing shows then there is no file present.
If it is there it should show up, have a look at the bottom left of your screen and if the file is there then you should see a Tab named BobCAD1 and also a Tab with the name of your STL file. Not sure why you would want a .STL file as you are not able to work on such files in BobCAD.