Forums Scripts & Popups help with a very simple script...

/aline [-ahilmnprs] [c] <@name> <text> add line to list
/cline [-ahilmnprs] [c] <@name> <N> changes the color of the Nth line to color C
/dline [-ahilmnprs] [c] <@name> <N[-N2]> delete Nth line through N2th line
/iline [-ahilmnprs] [c] <@name> <N> <text> insert line at Nth line
/rline [-ahilmnprs] [c] <@name> <N> <text> replace Nth line
/sline [-ahilmnprs] [c] <@name> <N> select Nth line

The c parameter specifies a color number for the line.
The -s switch selects the line that was just added and clears the current selections.
The -a switch selects a line without clearing the current selections.
The -h switch highlights the window's button if it's currently minimized.
The -p switch forces the line of text to be wrapped if it's too long to fit on one line.
The -r switch is used with /cline to reset a nickname color in a channel listbox to the default color.

The -i switch is used with /aline and /iline to indent a newly added line.
The -n switch is used with /aline and /iline to prevent a line from being added if it exists.
The -m switch is used with /cline when coloring nicknames in a channel nicklist, and makes mIRC also color the nick in channel messages.

If you are referencing a window which uses a side-listbox, you can specify the -l switch in the above commands to act on the side-listbox.