Go to the Machine Configuration (Setup / Machine Configuration) and click the Work Offsets item. To enter values into the work offset table just type a value into the appropriate entry and press 'Enter'. When finished entering values, save your changes and then click 'Apply' or 'Ok'.
To change the offsets from a program simply write values to the appropriate system variables. Note that NCPlot doesn't directly support G10 offset entry. For a list of the system variables see the help file under 'Variable Maps'
Hope this helps,