(this works the same for ISO and IMG files as well)

hdiutil attach /path/to/your/diskimage.dmg

When the .dmg is mounted, a message will display something like this (not that hditul will try to use the dmg filename as the directory name it creates in /Volumes):

$ hdiutil attach diskimage.dmg                                                                                           9:49
Checksumming Gesamte Disk (Apple_HFS : 0)…
...........................................................................................................................................
        Gesamte Disk (Apple_HFS : 0): verified   CRC32 $75753E80
verified   CRC32 $83E2BD54
/dev/disk2          	                               	/Volumes/diskimage

Unmounting a DMG from the Terminal is just as easy:

hditul detach /Volumes/diskimage

hditul detach /dev/disk2

If you are not sure anymore what was mounted where, then try using this to see a list:

hdiutil info

This will output something like this (depending on your situation of course):

framework       : 623.100.1
driver          : 623.100.1
================================================
image-path      : /Users/hans/Downloads/diskimage.dmg
image-alias     : /Users/hans/Downloads/diskimage.dmg
shadow-path     : <none>
icon-path       : /System/Library/PrivateFrameworks/DiskImages.framework/Resources/CDiskImage.icns
image-type      : UDIF read-only compressed (zlib)
system-image    : false
blockcount      : 32768
blocksize       : 512
writeable       : false
autodiskmount   : TRUE
removable       : TRUE
image-encrypted : false
mounting user   : hans
mounting mode   : <unknown>
process ID      : 52538
framework name  : DiskImages
/dev/disk2		/Volumes/diskimage