Underscores not supported while using java 7

Hi everyone,

I am beginner on java and i am currently reading and trying a tutorial.
I have an error on running my application that it should not appen ... see the capture. it will explain better than me :)
I don't understand why underscore is not allowed while i am using java 7 in my app ?!

The program telling me to use -source 7 but i don't know how to do :(
Thanks for your help