To create a new card, click on the category you want to create it in, then click "New Card" in the eWallet toolbar or Edit -> New Card in the menus. Cards always have to be created in categories, so the button will not be active unless you have a category selected.
The New Category toolbar button, and Edit -> New Category from the menus, will make a new category at the 'top' of your wallet file.
To create a category inside of another category, right-click (control+click) on the category you want to make a subcategory in, and choose "New Sub-Category" from the context menu that appears. You can also drag a category into another category to make it into a sub-category.