A family of Microsoft word processing software products for creating web, email, and print documents.
There are several solutions for the newer equation editor in the thread at http://answers.microsoft.com/en-us/office/forum/office_2010-word/equation-editor-matrices-in-word-2010/f31607e6-f49e-46eb-a176-97276e46a7cd.
I suggest you try the ones in my answer, the ones from Erdogan Cevher, and the one from 'bythewindow'. Also try the right-click menu items mentioned by IJMacD, although that adds only one column or one row at a time. Use whichever solution best fits your way of working.