I can use the Apple key and the + key to make type larger in a document or on a web page. The Apple and - key reduces size.
I use the Mac Mail software that came on the machine. OS-X 10.3.9
On my laptop, I cannot use the Apple + to increase email viewing. I CAN use Apple - to decrease it. I have to go to the Mail menus along the top toolbar and do a few clicks to make the fonts larger while reading mail.
When I try to do Apple + I just hear that annoying little "sorry you get nothing here pal " click.
Button down for a couple months if you can, OSX 10.5 (Leopard) will be out in October. Going from Panther to Leopard should be a nice change, and will hopefully resolve your + problem (if Arnold W didn’t nail it – Do laptops have a num-pad?)
Have you tried Apple and shift and “-” buttons? I often find that using the shift key will reverse the polarity of a lot of commands (i.e. make things larger instead of smaller, ungroup instead of group, etc…).