That's the problem with proprietary formats and the companies that use them.  They're like methadone clinics; they don't want you dead; they just want you to keep coming back to the trough.
The Navigatrix tool, 
gpsbabel, can translate many formats from one to another including many Garmin formats
- G7ToWin data files (.g7t)	 		 		 	
 - Garmin 301 Custom position and heartrate 	 				
 - Garmin Logbook XML			 	 		
 - Garmin MapSource - gdb	 	 	 	 	 	
 - Garmin MapSource - mps	 	 	 	 	 	
 - Garmin MapSource - txt (tab delimited)	 	 	 	 	 	
 - Garmin PCX5	 	 	 	 	 	
 - Garmin POI database	 	 				
 - Garmin Points of Interest (.gpi) 	 				
 - Garmin serial-USB protocol	 	 	 	 	 	
 - Garmin Training Center (.tcx) 
 
The Garmin format .adm is not one of them.
Some one suggested "
seaclear", which is in Navigatrix, but I've not looked yet.  Another suggested exporting from Garmin software to another format...one that could be translated with gpsbabel.
I'm about to test a Windows app called 
GPS Utility that claims to do adm.  I'll let you know what I find out.