This can be done via cell formatting, provided that you (and all persons who
use your Excel sheet on their computer) have a Unicode font (e.g. Arial
Unicode MS) installed.
Here it goes:
1) Format your cell with a custom format: #"u";-#"d";
This would render your value +50 as 50u and your value -50 as 50d
u and d are only placeholders for the "real" up- and down arrows, which you
will replace in a minute.
2) Go to an empty cell and from the main menu choose Insert => Symbol.
In the symbols tab set the font to Arial Unicode MS. You can also use any
Chinese or Japanese font that you find in the fonts list.
Into the "Character code" entry field at the buttom type the value 2191,
from the "from": drop-down box choose Unicode (hex). Now push the "Insert"
button.
Put in another character code 2193 and push "Insert" again. Push "Close" and
hit Enter key.
Your cell should now display an up- and a down arrow.
3) From the edit line of this cell select the first character (the
up-arrow) and copy the character with Ctrl+C
4) Go back to the cell that you formatted in step 1. Edit the custom format
by deleting the u and replace it with the up-arrow by pasting the copied
arrow with Ctrl+V.
5) Repeat steps 3-4 for the down arrow.
Cheers,
Joerg Mochikun