The lyrics need to be entire english words, which can't be broken up into syllables.
If you want the result to be evenly divided notes, you can add "/u" to the end of the word.
Make sure that there are notes (whether they have lyrics or rests) before and after your selection.
Most importantly, you must be using an arpasing voicebank. The assistant works by reading the top level oto.ini file of the currently selected voicebank. If you're not using an arpasing voicebank, or if the voicebank has any non-standard extra samples in the main oto.ini file, the assistant will not work.