Inserting symbols into my xcel worksheets

Question

For some reason neither my 2003 nor my recently acquired 2007 xcel software will let me insert symbols into my worksheets. I use statistics a lot and need the greek letters for sigma and mu (among others) in the text describing various quantities. I used to be able to insert symbols with no problem. For some time now when I go to the symbol menu to select a symbol, I click on the symbol and nothing happens.

Cannot understand what has happened to thwart my symbol subroutine.

Thanks,

Benny Jay