Hi,  Iam not quite clear on your question, if you are migrating your PIX 525 to ASA5500, ASA platform should have come with ASDM software already installed.  
If you are upgrading PIX525 say from code 6.x to code 7.x and above refer to this link for information, various code have different ASDM versions so go over this link for more information on PIX.
PIX 500 series code upgrade resource
http://www.cisco.com/cgi-bin/tablebuild.pl/pix
ASA5500 code upgrade resource includes ASDM information.
http://www.cisco.com/cgi-bin/tablebuild.pl/asa
Rgds
Jorge
					
				
			
			
				
	Jorge Rodriguez