Make sure that the ROM is pre-rooted.
Once you input "su", your phone should display to you that adb is asking for permission depending on your setting of Superuser. You should allow it to proceed.
Edit:
It'll be best for you to update Superuser and its binary first hand.
Sent from my HTC Desire S using XDA App
i have some problem this i read it need pre rooted but how to peeroot rom
i have done unlocked and goldcard