To launch ASDM or IDM the basic requirement is that your Java maximum heap size must not be lesser than 256mb. Adjusting the maximum heap size is easily done through JRE plugin, however this works fine for JRE version 1.4 and 1.5. For JRE 1.6, it has no effect even if you have changed the maximum heap size to 256mb.
Most people resort to downgrade their Java plug-in to version 1.4.2_19 (the last release for the 1.4 train) and it worked. You do not need to downgrade your Java plugin anymore, this tutorial is meant to solve your current problem of unable to launch asdm/idm.