After spending a couple of hours scratching my head- trying to figure out what's stopping my Vsphere to install an OS from an ISO, I finally found the answer.
The DNS IP on client machine was different to what the Vsphere host had. Once I adjusted my client DNS, issue resolved & happy days.
Check DNS IP on host in configuration>DNS and Routing and you can edit by clicking on properties..
DNS on client can be checked/edited from LAN settings..
If you are trying to install an OS from ISO from a client and having similar issues then it is worthwhile checking the above...
It is unlikely that the dns configuration caused this, its very common to forget to check the box "connect" on the iso.
it could be a DNS issue if the client machine is accessing a DNS server that doesn't have records for the hosts. The way the 6.0 (and possible 5.5) remote client through the web client works is over the fqdn. So you might be accessing the host by IP address but when you remote to a guest or try to attach a client ISO its using a FQDN address instead of some sort of routing through the client directly, its weird and i'm not explaining it properly but I ran into a similar issue last month on a 6.0 install.