It's a similar process to ESXi 4, but the names have changed. kb.vmware.com/kb/1017910
In the DCUI, go to Troubleshooting Options and enable either ESXi Shell (Local Tech Support Mode) or SSH (Remote Tech Support Mode).
You can also do this in the vSphere client - Configuration > Software > Security Profile and click the Properties for Services. Then start ESXi Shell or SSH.