In ESX 3.0 you can only access ISO Files in /vmimages or on VMFS Storage.

To get alround this just mount the Network share into the /vmimage directory (if it doesn't exist just create one)

To mount a windows share goto

If you placed your ISO's in the directoy  /isoimages the yu just add a link to /vmimages:

Create a symlink in /vmimages that points to /isoimages. Here is the command: ln -s /isoimages /vmimages/iso