Good macro lens for Nikon D5100?

At the moment I'm using a reversing ring with some old Miranda lenses (for Olympus OM10) and i managed to photograph this: http://500px.com/...er_library

The picture is ok but as you can see its not in great detail.

So i want to get a decent Macro lens for my D5100 that is not overly expensive and can take pictures like this: http://500px.com/...?from=user

Using a lens reversal ring is not easy. For a start there's no control of the aperture, which is why your image has such a shallow depth of field.

Buying a dedicated macro lens is a great idea, but it's not the only option. You can also use extension tubes, some even have electrical contacts inside so you can control the aperture (and therefore the depth of field).

The problem you have with ANY macro shot, whether it's with a reversing ring OR a dedicated macro lens is the incredibly shallow depth of field. That's the problem with your shot, not lack of detail.
This is because of the very close camera to subject distances, (that's one of the factors affecting depth of field).
The other image you linked to, I suspect, is an image 'stack'. You take several images at different focus points through the image (like 'slices') and then use software to stack them together. This gives you a depth of field that would be impossible to achieve with a single image.
Helicon Focus is the most well known software for this; http://www.heliconsoft.com/heliconsoft-p…
THis is a shot of mine with a dedicated macro lens (Sigma 50mm f2.8) and no focus stacking - as you can see, there's still a problem of shallow dof.