First of all, make sure you have saved the number in your phone book, since it is from there that Whatsapp shows the contact list. To do this, click Contacts in the application list (you get to it by sliding your finger left on the home screen). Once there, look for the contact by clicking on the magnifying glass icon at the bottom of the screen. If not ... save it!
Once you have verified that you have saved contact, open Whatsapp clicking on its icon in the application list.
Once Whatsapp, swipe left on the screen to switch to the tab "Favorites".
Click on the magnifying glass icon at the bottom of your screen and look for the name of your contact. If not, chances are that Whatsapp has not yet updated the list because it takes very little contact in your phone.
Update the list manually: Favorites, click on the three dots that appear on the bottom right of the screen. Among the new options that appear, click on "update". The new contact should be added to the list of favorites.