You can add text to an influence by right-clicking on the influence and selecting Add Label from the context menu, or Shift+double-clicking on the influence (double-clicking on the influence while pressing the Shift key). This will add a text box to the influence with the word "Text" inside (which you can subsequently replace with your own text).
You can edit the properties of the label (e.g., font, color, outline) by right-clicking on the influence and selecting Properties.... This dialog is similar to that used to edit the properties of text objects in the graphics pane The primary portion of the GoldSim interface, where the graphical depiction of the model is shown..
To delete the text associated with an influence, select the text box, and press the Delete key.