I don't have that package, but can make a pretty good stab at what's going on here.
From browse mode, the mode FileMaker opens in, you can only add fields if you are using a layout set for Table View by clicking the modify button. If it is in Form or Layout view, you can only add fields by entering Layout Mode and using the field tool to add them. You can only add fields this way if they have already been defined so if this is a completely new field you want to add, first open Manage | Database | Fields and define the new field there, then return to your layout and add it to the layout if FileMaker didn't add the field for you. (There's a preference setting for automatically adding new fields to the current layout that may or may not be enabled.)
If that doesn't answer your question, feel free to use the Post a Answer box to respond with a more detailed description of what you want to do here.