We will try to help you.
You should be able to. To make sure, go to the Apple icon in the upper left corner of your screen, click on About this Mac and look at the specs for the macbook you have. Then go to the Apple site and check the minimum system requirements to run your mac. If your system is the same or better (for example, if it lists 512 MB of RAM and you have 1GB, that means that part is better than needed on your system), then you can install . Insert the OS X 10.4 CD and in the pop up you will have an option Upgrade. If that option is not available you will have to buy a copy of 10.5 from Mac store or somewhere else.
Robert you left and I couldn't continue, So I waited and got help from Apple. I just needed to up date my soft ware to 10.5.