[dacoobob]

Look under Modifiers tab->Adjustments->Temporary Adjustments. In the list of adjustments search for "Circumstance Bonus". After adding it and closing the menu, click the newly-added adjustment (in red text) to open it and select what it applies to (Diplomacy, Will Save, Strength, whatever) from the dropdown menu, and set the size of the bonus in the box above.

And voila! Now the circumstance bonus will appear on the Play tab, in the Modifiers pane under "Temporary Adjustments". It even includes a checkbox to turn the bonus on or off whenever you need to, right from the Play tab.