Adding a word to the dictionary

[Christopher_Dureau]

Version: 2008
Operating System: Mac OS X 10.5 (Leopard)
Processor: Intel

I have recently acquired a Macbook and moved from Windows to Mac. I use lots of new words not found in the current dictionary. When I try to add the word I find that this opportunity is not available to me. The word 'add' on the spelling menu is faded (not available). How do I turn this facility back on. How do I add a new word to the dictionary?

Thanks

Chris