Celestia doesn't display those values directly. However, Celestia can display the distance from the viewpoint to a specified object. So one can determine those values by setting Celestia's time to the time of either aphelion or perihelion, placing the viewpoint at the center of the sun, and Selecting the comet.
The orbital parameters which Celestia uses and which you can easily see are those in whatever .SSC catalog is used to define the orbit of the comet that you're interested in. Given those parameters, one could write a Lua (Celx) script to calculate the aphelion and perihelion of a comet. I don't think anyone has written one yet to do that, though.
Note: Apogee and perigee are the distances from the center of the Earth to an object orbiting around the Earth. The suffix helion is used for objects orbiting around the Sun.
p.s. I suspect Celestia is not the best program to use to determine the best times for observing cometary flybys. Unless the objective is to learn how to calculate such things, you might want to consider using JPL's Horizons ephemeris server.
http://www.ehow.com/how_5522248_calcula ... rbits.html briefly describes a procedure for determining the info you're interested in, although it doesn't provide the specific details.