Hi Eva,
Hmm, what version of Mac OS? Recent versions (as in last several years)
of Psi on Mac support voice calls and the feature should always be
present. If you ctrl-click on a contact, "Voice Call" should be in this
menu.
The one gotcha is that you almost always need to tell Psi to use a STUN
server in order for the call to work. It is in the account settings, and
should be set on both sides.
This page has a list of free stun servers:
http://www.voip-info.org/wiki/view/STUN
Justin