v -t time -e sim_annotation time unique sequence id text to display
For example:
v -t 4 -e sim_annotation 4 3 node 0 added one mark
This line calls a special tcl function sim_annotation in nam, which inserts the given string node 0 added one mark into nam's annotation pane. Look at Animator instproc sim_annotation in tcl/annotation.tcl for the implementation details.