Virtualbox Backtrack 5 Windows

Download VirtualBox (Old Builds): VirtualBox 5.1 The Extension Packs in this section are released under the VirtualBox Personal Use and Evaluation License. All other binaries are released under the terms of the GPL version 2.

I'm trying to install backtrack linux on virtualbox on windows 7 as the host machine. I'm trying to follow this tutorial (written for mac): I downloaded backtrack 5 R2 64 bit and I'm trying to install it. When I power on the virtual image, i get the following error: VT-x/AMD-V hardware acceleration has been enabled, but is not operational. Your 64-bit guest will fail to detect a 64-bit CPU and will not be able to boot. Please ensure that you have enabled VT-x/AMD-V properly in the BIOS of your host computer. I then continue with installation.

It eventually tells me that I'm using an i686 cpu and that I need to use a 64bit cpu. Download tutorial gita slank.pdf. I have a 64bit core i5 cpu. What's the issue?