Using a datastore ISO
1) Start the vSphere client, on the host's summary page right click on the datastore and select Browse Datastore.
2) Once you have the Browse Datastore window open you can copy the ISO image to your datastore.
3) Edit the VM, select the CD, choose the datastore option and find your ISO image. Remember to check the option "Connect at power on".
4) Start up the VM and boot from the ISO image.
Using a client ISO file
1) Set the VM to use the Client Device CD option and power on the VM.
2) Open the console for the VM (assuming you have no guest OS installed, you'll get to an error that states that no OS is installed).
3) Click the CD icon and choose to connect to a local ISO image (you can also choose the local CD device on your management PC).
4) Click on the VM's console window to make it active, and press any key. You should start to boot from the ISO image.