Host in HA Cluster must have userworld swap enabled

Error when enabling HA on ESX 3.5 i servers : Host in HA Cluster must have userworld swap enabled

This error you get from a lack of storage reservation for saving the HA settings on the ESX3.5i. You can set this using the following procedure.

To enable swap on your ESXi 3 host system:
  1. On the VirtualCenter Server, select the ESXi 3 Server host.
  2. Click the Configuration tab.
  3. Click Advanced Settings.
  4. Choose ScratchConfig.

  5. Configure ScratchConfig.ConfiguredScratchLocation to a valid directory with sufficient space (1GB) to hold the userworld swap file. The userworld swap can be configured on local storage or shared storage.

    Note: Each swapfile needs a unique name across all ESX hosts.

    For example, /vmfs/volumes/.

  6. Select the ScratchConfig.ConfiguredSwapState option.
  7. Click OK.
  8. Reboot the ESX Server 3i system.
After you have enabled swap on the ESXi 3 host system, you can add the host to a VMware HA cluster.

Source : from the VMware Site