The likely issue is that your mesh is not tessellated enough for it to "pick up" the bump map details. I'm assuming here that your square only has 4 vertices (or 8 vertices if it's a squished cube?).
You have to remember that geometry modifiers are only running once per vertex (and not once per fragment as is the case with surface shaders). If you replace your square by, say, a 256x256 divided flat plane, or modify your cube so each face is something like 256x256 polygons, then you should start seeing the effect of your displacement map.
For best effect, you may also need to multiply the map's values, but that will depend of course on your data.
Please remember that tessellating your mesh as mentioned above will significantly increase their sizes, and will also increase the processing time. So try to keep the tessellation level as low as possible.
Geoff