Missed a few days this week, but I think that it's okay to blame the homework and my birthday. I already posted one sed onliner dealing with the replacing of text. This one should append a new line to after a line that matches a sed script expression:
sed '/FINDME/ a\
The new line we are adding` fileToEdit.conf
The -i switch can be added to make this edit occur "in-place" (homework for the interested reader).
The new line is added after EVERY line matching the expression, in this case FINDME. I might get around to adding a part two to this, where you can append after only a single specific line, regardless of multiple matches. One way to do this would be with the ";" operator. However, I am getting back to my review assignment for school. Maybe tomorrow I will do this, or some Perl (yeah!)