Arduino Leonardo and 1Sheld.

Projects General Guidance
1

I am using Arduino Leonardo and 1Sheld.

At what voltage should I keep the switch in the 1sheld (3.3 v or 5 v) ?

Thanks

2

You are never going to make it as an engineer, unless you learn to convey as much information as possible. Things like links to the products in question are damned useful, you know.

3

Did you mean "1Sheeld"? http://1sheeld.com/

Did you try reading this?
http://1sheeld.com/tutorials/using-1sheeld-with-different-arduino-boards/

The Leonardo runs at 5V so you should run the 1Sheeld at 5V.