If you're getting this host error from the browser:

HTTP: processing GET URL '/admin/public/index.html'

First run: show flash

You should see the asdm image there.

If you don't, you'll need to use tftp and grab the image from another ASA or from Cisco if you have a service contract. Make sure it works with the compatibility matrix: http://www.cisco.com/en/US/docs/security/asa/compatibility/asamatrx.html

Enable mode: en

Configure terminal: config t

Make sure the current configuration makes sense: show run http (This is an access list)

Enable the server: http server enable

Designate the ASDM image file: asdm image [location of asdm image file]

Write to memory: wr m

Reload: reload

If that doesn't work the following might help: ssl encryption aes256-sha1 aes128-sha1 3des-sha1