Probably while creating the new virtual computer you did not select Windows7 64bit. Create a new one and make sure that you have selected Windows 7(64 bit) from the drop down box.
It can also be that virtualization is not active in your computer or that your simply does not support it, to check if you system supports virtualization run this command
grep ^flags /proc/cpuinfo | head -n1 | egrep --color=auto ' (vmx|svm) '
and look for
svm, if any of those shows up your system supports it.
If it does and you have selected the right template for the virtual guest then probably your BIOS has it disabled.
Have a look in your BIOS options or drop a comment in your question mentioning which computer model / motherboard you have.