I don't know if you can add this to an existing picture, but the next time you have a need for craters (and I don't know how much of this you can do with PS 5, either) but start a new photo, filter>render>clouds, then duplicate the image, save that as cloudsimg2, go back to original filter>render>diff. clouds, then bring up levels control (ctrl+L) slide input levels to 0/1.00/>150< and output >115</255 (you can play around with the numbers in the > < ).
New layer, blending mode to overlay, gradient tool to radial set it to Foreground to Transparency preset, paint some black areas, then some white areas. Add a new layer and use this
custom gradient (attached to post) put in some rings and play with the settings of the gradient to come up with some varying craters. you can blend the rings into the the terrain if you like by using filter>render>diff clouds. And to give the craters a natural feel you can go to Render>Distort>Displace (keep the default settings of 10) and select the psd file you created in the begining of the steps...
Then select your layers and ctrl+E to merge them, rename the layer craters, then Filter>Sylize>Emboss with an angle of 135, height 4 and amount 248%..... that should give you some nice craters on your landmass....
If you want crisp craters like on a battlefield, I imagine that not adding the Displace step would do the trick (but I don't know, I haven't tried it yet. I could turn this into a full tutorial I suppose if I really needed too.