Here are two simple macros to do what you want. If you want the same macro to add/subtract depending on what the user chooses, you'll have to add some coding or (preferably) create a userform.
Sub AddtoCell()
oNumb = ActiveCell.Value
uNumb = InputBox("Enter Number to Add", "Add Number")
nNumb = oNumb + uNumb
ActiveCell.Value = nNumb
End Sub
Sub SubtractfromCell()
oNumb = ActiveCell.Value
uNumb = InputBox("Enter Number to Subtract", "Subtract Number")
nNumb = oNumb - uNumb
ActiveCell.Value = nNumb
End Sub
Assign these macros to hotkeys or to toolbar buttons for quick access.
-Ben