Hi,

I would make sure that the BIOS is up to date. Which CPU do you have?

That should work. Try powering the server all the way off, sometimes just a warm reboot will not switch the virtualization setting. 

Are you using our image or the Vmware one?

BIOS version was 4.2.1 but it showed this error. 

On Dell website it shows the latest BIOS version is 4.1.1 for ESXi. So I downgraded to 4.1.1 and tried to install the ESXi 5.5 downloaded from Dell website. Then again the same error appears.

Processors: dual core opteron processor 2222se

Does this support nested virtualization? 

I have tried power on/off even by the physical power supply many times.. it does not work. 

In BIOS settings it shows Virtualization Technology is enabled but ESXi does not detect a virtualization technology.. what should I do now? our application needs ESXi to run on without problem..

Your image for ESXi 5.5 

What does the vmware cpu tool show? www.vmware.com/.../shared_utilities.html

This is what is shows. Is it ok? Does this support nested virtualization too? what can be the wrong?

That seems to show that virtualization is working properly. Can you try an older version of esxi and see if it will let you install?

I tried to install ESXi 4.1. Only this version didn't show the error during installation.

But when a VM host which needs nested virtualization starts  its shows "Intel-VTX of AMDv is not found"

Also what i need is ESXi 5.x which i have a license key.