Opening/Mounting a .img file
Provided the image is a correct image file supported by OS X the correct command to mount would be hdiutil attach imagename.

Not all files ending with .img are disk images at all, let alone supported by OS X. You can try to use file image name to see what this file actually might be, but even for valid classic image files this may just result in data as an answer.

It would be most helpful to know how or using which software this image was created.