Inserting Segnos is done through the symbols dialog.

Segno should be the second item on the first row.  Once you've arrowed onto
it, press the space bar to select it, tab round to the radio buttons and
choose "attach to system" and finally tab to the OK button.  Don't forget to
press space on the symbol, if you don't then the first symbol on the first
row will be added to the score and this symbol is just an empty graphic.

Codas are a little fiddly because there are a few things to do.  In brief:
. a double bar line is placed before the coda starts.
. The first bar of the coda and the bar before it are separated by a small
gap.
. The word "coda" or the equivalent symbol is placed above the first note or
rest of the coda.
. A text direction is put at the end of the bar before the coda to tell the
player what bars to replay.

Adding  a double bar line before the coda
1. Select the last note of the bar before the coda. (Sibelius will add the
double bar line after this note.)
2. Choose Create > Bar line > double
3. The menu will close and the double bar line will be selected.  Do not
move away from the bar line because the next steps need it to be selected.

Creating a gap between the double bar line and the start of the next bar.
1. The double bar line should be selected after you created it.  If it isn't
then press Alt+F3 to open the system items dialog, find the double bar line
you just added and press enter.
2. Choose Layout > Break > Split system.  (Do not choose system break.  This
puts the first bar of the coda on a new system.  All we need to do is put a
gap between two bars and this is what "split system" does.)

Adding directions for playing bars before the coda
1. The double bar line should still be selected after you created the gap.
If it isn't then press Alt+F3 to open the system items dialog, find the
double bar line you just added and press enter.
2. Choose Create > Text > Other system text > Repeat.
3. From inside the text box press Applications, Shift+F10 or
RightMouseButton.
4. From the menu, choose the text you want.  You can enter the by hand but
if you want Sibelius to play the score properly you'll need to take care of
using the correct capitalisation.

Adding the Coda
1. Go to the beginning of the first bar of the coda.
2. Press Alt+Control+T to insert a tempo direction.
3. From inside the tempo edit box press Applications, Shift+F10 or
RightMouseButton and from the menu choose "coda".

There is a symbol for coda which you can use instead of the word which is
also available from the menu, but JAWS can't identify it.  A sighted
assistant could tell you which item it is in the menu.  I believe it also
has a shortcut key which they could tell you as well.  Personally, I prefer
to use the text because it's clearer.  The coda symbol is certainly an
interesting looking one but there's too much symbolism in life as it is.