Hi Kitty,
When using merge fields on particular screens, i.e. the New Activity screen, you would need to use the table which that screen was built off of. For example, if you want to send out an email to a contact from the New Activity screen you would use Activity.ContactName as the merge field since the screen is based off the Activity table, and ContactName is the field within that table which holds the value of your contacts name. Go to Home > Email Templates and look at the merge fields used for those templates, they won't all be the same because the tables and field names are different depending on what screen the email is sent from.
For a little more info on the List Builder and merge fields check out our List Builder Help Centre Article, in particular the Automatically Merged Fields section.
- Ashur