iostream is a C++ header file that should be installed by your compiler suite (usually in a sub directory of the compiler suite directory named
include).
When compiling source files, the compiler will check specific directories to locate include files. These directories must be configured in the project and/or general settings.
I don't know the DM compiler but according to a quick web research there is a configuration file named
sc.ini in the
/dm/bin/ directory. This file should contain a list of pathes to be searched for include files (see also
DMC - Digital Mars: sc.ini[
^]).
The pathes may be also specified using an environment variable (see
DMC - Digital Mars: Environment variables[
^] )
Ensure that the directory containing the
iostream file is listed in the
INCLUDE
section of the INI file or the
INCLUDE
environment variable.
If there is no
iostream file in a sub directory of the compiler check the installation (it may be incomplete or broken).
For further information check the compiler documentation.