To add to what Dave says, creating a new user control doesn't do anything useful on it's own either:
usercontrol user = new usercontrol();
user.stringA = "HI";
Unless that instance is added to the Forms Controls collection (or the Controls collection of a control on the form already) then it isn't displayed at all.
Probably what you need to do is access the control instance you already have on your form - just use it's name - and set it's property as Dave showed you.