This is an old post, but still showing up at the top of searches so I wanted to add my 2 cents about how to get that look without using a $200 plugin.
There is no such thing as a metallic color, what makes something look metallic is how reflective it is. To mimic this digitally you need to punch up the contrast so that you have deep blacks and "shiny" whites and grays.
In Photoshop CC, and most previous versions, do a high pass sharpen layer to bring out detail (duplicate your layer, then Filter>Other>High Pass, adjust so that edges are more clear, then change blend mode to Overlay). Create a Black and White adjustment layer (optional) to set the contrast where you want it, lightening trees or skin tones, darkening the sky, or whatever makes the image look best to you. Next create a Gradient Map adjustment layer, and set it for a simple black and white gradient (black on one side, white on the other, nothing in between). Pull the black clipping slider (the bottom black slider on the gradient) towards the center until you are satisfied that your blacks are dark enough. Next, Pull the white clipping slider towards the center until you are satisfied that your whites are "shiny" enough. Finally, move your neutrals (the dot in the middle) to adjust your mid-tones. Usually you will want to move it towards the white slider so that you make your grays more "shiny."
As a final step, you can add tinting by making your image a duotone or tritone. A little sepia, copper, or gold (dark yellow) tinting can warm up the image, and shades of blue will cool it down. Most metals are not color neutral, so just try to picture what kind of metal you want to mimic.
For printing, you do not need to do anything to get a metallic effect except for have your print made on metallic paper. Metallic paper and printing processes make the surface reflective, so you have no need to mimic it in Photoshop. Just remember that a true metallic print looks best when made from a high contrast file (in my opinion), so getting your file as close to the desired end result as possible will most likely result in the print looking how you want it. I would at least advise making sure that you do a black and white adjustment layer, levels layer, and curves to punch the contrast a bit.
Good luck.