To use a Vocaloid trial, you have to download and install it. The trials that are currently available:
SF-A2 miki, Kaai Yuki, Hiyama Kiyoteru, and Nekomura Iroha have 14-day trial versions available on AH-Soft's website, however if you do not own VOCALOID2 in English VOCALOID2 demo will be in Japanese.
And MAYU has a trial available from her official website.
MAIKA also has a 14-day trial avaliable through her official website.
CUL, Lily, kokone, Gachapoid, Chika, and all of the GUMI Megpoid APPENDS (excluding english), have 14-day demo trials available from the linked sites. However, they are limited to how long they can sing.