I use ESX 3.5, not 3.5i, so I did some digging and found this info:
ESXi 3.5 does ship with the ability to run SSH, but this is disabled by default (and is not supported).
1) At the console of the ESXi host, press ALT-F1 to access the console window.
2) Enter unsupported in the console and then press Enter. You will not see the text you type in.
3) If you typed in unsupported correctly, you will see the Tech Support Mode warning and a password prompt. Enter the password for the root login.
4) You should then see the prompt of ~ #.
Hope that helps.