Hi,
You have to use "local variables" to read and write data to control.
To create local variable you have to right click on the control node in the diagram and select Create->Local variable from pop-up menu. The new local variable corresponding to your control will appear. You can change this local variable to write or read mode by right clicking on it (select "Change to write/read). In write mode you can write data to the local, and in read mode you can read data from the local. The local variable is connected with the control on your front panel. So when you write data to the local variable - you write them to the control, and the same thing with reading.
Read help about local variables in LV help file.
Good luck.
Oleg Chutko.