If you're copying the DMG to the client workstation and opening it, you probably then will need to take control of the workstation and run the installer from there. Most such installers require some manual intervention and so can't be run from a command line.
If the DMG contains the software as a Mac OS X package, use the Install Packages command under the ARD Manage menu and push the package to the workstations that way. If the DMG doesn't have a standard package but you have a number of workstations that you need to install on, you may find it easier to build your own package. You can find some information on this in the ARD Administrator's Guide, and there are third-party tools such as
JAMF's Composer that can help you build a package more easily.
Regards.