adding files to pre-existing CD with files on it

I have Windows Vista---how can I add files to a CD that already has files on
it?
I have tried the HELP which was no help---it just told me that I could do
it. I have
performed this task before on my other computer with XP and had no problem.
I just tried to add files to a CD and it went thru the motions as if it
added them, but when I checked the CD afterward, they were not there! Please
help! thanks!