You can run 32 and 64 bit Windows on the same host / cluster without any issues. If you haven't run any x64 VMs yet, it may be the case that Intel VT is not yet enabled. You can check by running the below command. Your output should be the same. If not, then you'll need to restart the host, go into the BIOS and enable the option (it'll be listed as Intel VT / Virtualization Technology / etc). After you save the setting, it's a good idea to do a cold restart on the host. After you've made that change, the command should then have the below output and you'll be ready to create x64 VMs. You won't have to make any other changes to ESX.
esxcfg-info | grep "VT"
|----VT Support............................................3