Select all the text (CTL A) and click on the file menu: Format > Asian Layout > Phonetic Guide.
The selected text will have phonetics, and there are a few options for positioning them and changing their size.
If you are seeing Zhuyin phonetics and would prefer Pinyin, (or vice versa) you may need to change the formatting of the Chinese characters.
Select the text, click on the file menu:Format > Reveal Formatting. In the sidebox that pops up, you can change the language settings to PRC for Pinyin or Taiwan for Zhuyin.