Click in one of the cells and then go up to the Table Menu and choose insert and then select Columns to the Left or Columns to the Right depending on where you want the column to be inserted.
I am using Microsoft Word 2007.
At the moment I have a document which contains a 2 column table, in the format WORD | MEANING. What I would like to change it to, is WORD | MEANING | WORD2 | MEANING2, so that in a single page, I could fit twice the ammount of entries.
How can I do this? I thought the columns button under Page Layout would work, but it doesnt :(.