It took me a while, but i think I found it (I guess it doesn't work if there are merged cells).
"Choose Hide Row or Hide Column from a row or column reference tab’s pop-up menu."
The same instruction is written within the Help file for Numbers. The problem was that I didn't know what was meant by the "reference tab's pop-up menu". I wrote that there should be a picture of what is meant, just to help people (like myself) identify the position of this "reference tab's pop-up menu". I guess my next question would be if it's possible to do it if there are merged cells?
The question is handled in the following topic, but no answer has been given yet:
http://discussions.apple.com/thread.jspa?messageID=7876554�
(For those of you who still don't get it, the "reference tab's pop-up menu" is the little downward arrow triangle that appears next to the Column/Row when the mouse hovers over the Row Number/Column Letter. For example, to display the pop-up menu for Column B, place the mouse over the letter, and next to the letter, you will see the small triangle.)
Thank you very much for your help. If you know the answer to the merged cell question, that would also be greatly appreciated.