How to write an ADM file? 

It is very important to understand the ADM language for GPO Admins. At least you can fix an error with ADM. Find the below links for How to write a ADM file. Now a day ADMX/ADML is used for windows 2008.

ADM Location: "%SystemRoot%\Inf" folder
ADMX Location "%systemroot%\PolicyDefinitions" folder

You can convert ADM to ADMX, download the below Tool.

Managing IE cookies through ADM

Download the Mozzila Management ADM

Download Google management ADM, Its awesome.