What has changed in the way that Fields and Calculations are done?
In Extender 2013, you will be able to set all the Field and Calculation options at once.  To create a field, you will just need to select the field position and then select the Add Field or Add Calc button.
Within those windows, you will be able to enter the Field Name as well as setup the type and any additional setup that is available for that type such as the decimal places, list entries, and calculations.
Below is what the Add Field window will look like in Extender 2013.
Below is the window you will receive when you click the Add Calc button. The setup in the Add Calculated Field window has not changed between Extender 2010 and 2013.