You would use the 2dlabels command to show text on the frames as the trajectory plays.  The first thing you would do is create a 2D label that will be modified each frame, via a command like:

2dlabels text start xpos 0.1 ypos 0.1

You should check out the help for the 2dlabels command to get the size/color/position you actually want the label to have.

After that this command (well, multiple commands but all on one line) will show the frame number and record the movie:

movie record; perframe "coordset #1 $1 ; 2dlabel all text 'Frame $1'" frame 23; wait 23; movie encode

So first, if the trajectory isn't model 1, change the "#1" in the coordset command to the appropriate model number.  Second, the two occurrences of "23" should be changed to however many frames your trajectory actually has.  Third, in case sending this by mail munged the quote characters, the outside quotes around the coordset and 2dlabel command are double quotes, and the inside quotes around Frame $1 are single quotes.  Lastly, the "$1" occurrences will be replaced with the frame number as the perframe command executes, which is why the 2D label will show the correct frame number as the trajectory plays.  You could of course change "Frame $1" to some other phrasing as you prefer.