When you press the book button on the Clue Editor toolbar a menu pops up with a list of available look-ups which are available. You can also press Ctrl+L. By default, there are Letter Indicators and Anagram Indicators and a couple of others. When you select an option another menu pops up with the available words. Selecting the word on the menu pastes it into the clue editor.
You can edit the letter and anagram indicators displayed by editing the ANAGINDS and LETIND files supplied with Crossword Compiler. Edit them using Notepad. The format of the file is
MENU <Menu caption>
First on submenu
Second on submenu
MENU <Second menu caption>
First on second submenu
etc...
The menu captions can contain the '&' character which is the keyboard shortcut.
You can add another lookup list by selecting the Custom Lookups page from the Preferences dialog box.