As far as I've been able to find out, there is no way to make them visible all the time.
If you double tap on a cell to enter the editor, and then choose the = sign to enter formula mode you can see the labels. This is probably to make editing formulas easier. It is the only way I've ever seen the letters and numbers.
No problem. I was kind of anoyed at the omission, until I realized that the only time I realy needed them, they were there. Numbers does quite a few things differently, and it has taken a while to get used to them.