Adding someone to your crush list means an indirect expression of your romantic expressions. These are for your personal use only. If you add someone to your crush-list and the same persons accepts your request, you will both receive a message indicating that there is a match. The rest is up to you!
But this option is only available if you have already filled your personal page. This is how it is done:
1. Navigate to a profile and click on More link from left sidebar
2. Click on add to crush list tab to add the person to your crush list
3. You will see a message once you have added the person successfully.