you have to export an image. Basically once you make your 3d model you have to render it as a .jpg or a .targa or something of that sort (a 2d image) and then take that into photoshop.
To do this go to Render > Render Settings
change settings such as output type, location ext... Then go to Render > render in picture viewer, and once that is done go to file > save and save it to a location with a 2d image extension (.jpg / .targa ect...)
Now open that into photoshop.
Note if you want to get a very good render with no background, save it as a targa with an alpha layer. Then when you open in photoshop go to chanels > cntl + click your alpha layer to select your render and then copy and past into another file.