What is odd to me is it seems like you are telling me you were able to use it at one time.
There are a couple of ways around this. First have a look at the computer itself for a microsoft sticker with the key for office, you may also see a windows key but you want one that says office. Sometimes they will stick it on the computer itself.
If the key was entered at one point on this computer.To reveal all product keys ever used on the computer that were installed and activated, use this free program, click here: http://tinyurl.com/2efkzu7
Install the program, make sure when you see the screen below you choose custom and uncheck the boxes beneath as you do not need any of that.
After it installs run the program and if it can find the key it will display it. Basically it will search for all software keys.
If not, Please click here: http://support.microsoft.com/kb/2548260
and make sure that you have done all that is listed here.
If none of the above work you will need to call Microsoft directly for a replacement key.
I believe you are in New Zealand so the number for Microsoft would be 0800 800 004
If you need further help please do not hesitate to reply back and let me know.