You can open the Custom Addressbook search dialog using Menu: Items⁄Search, the 'Search' button on the toolbar or you can type Ctrl F on your keyboard. Inside the search screen, just type the text you would like to find in the field Text to find and click Find.
By default, the search dialog will use the entire file (meaning it will search all categories and attributes). If you click on the button 'Options' inside the search screen, you can limit your search to a certain category and⁄or attribute.
Moreover, the option Find text at start of fields will only return those items which contain the Text to find at the beginning of an attribute (or field).
The option Match case will make your search case sensitive.