If all times entries are showing zero amounts, it is possible that you did not enter hour hourly rates during the registration or during adding staff.  Click on "Staff" tab on the top side of the screen and edit to enter your global hourly rates.  You will then need to edit your old time entries manually.

However, if you want to record hours but do not want to charge client for those hours, in Add Note, enter hours/mins (or tenth hours) and then manually change the amount in Total Cost box to zero.  On invoice this entry will show up as "No Charge" entry.