I just downloaded ImageJ (actually, I installed it from the Ubuntu software centre) and the picture in the question you linked to, tried following the instructions given in the answer and got a pretty nice-looking result.
Here are the exact steps I followed:
- Start ImageJ.
- File -> Open..., select image to be fixed.
- Process -> FFT -> FFT
- Select pencil tool from the ImageJ toolbar window.
- Edit -> Options -> Line width..., set line width to 10. (A slightly larger value could probably have worked, too.)
- Use the pen tool in the FFT window to black out all the regularly spaced bright points (except the really big one in the middle).
- Process -> FFT -> Inverse FFT
...and that should do it.
Now, one thing to keep in mind is that, apparently, when you select an operation from the Process menu, ImageJ applies it to the image window that you last selected. If you try to use Process -> FFT -> Inverse FFT when you have a non-FFT image window selected, you will indeed get an error message saying "Frequency domain image required".
I suspect this is what went wrong when you tried this. Just try again, and this time make sure the FFT window is selected when you use the inverse FFT, and it should work.