Change value of javascript variable with NGUI button

I have a NGUI button that I would like to have change the value of a javascript variable when clicked. My javascript variable is named BeginMission and set to "false" in a javascript called Mission_Control.js I have attached a C# script called BeginMissionBtn.cs to the NGUI button. Since I am new to scripting I am confused as to what my code in BeginMissionBtn.cs should be to change the value of BeginMssion to "true" once the button is clicked. I know I should use the OnClick function, but I don't understand how to address/access the javarscript variable's value. Could someone help out with this please.