Open your Word 2007 document and click the Page Layout tab on the ribbon.  Click on Line Numbers and then Line Numbering Options…

 

You will now get The Page Setup dialog window.  At the bottom of the page click on Line Numbers.

In the Line Numbers dialog box place a check in the Add line numbering box.  Here you can select spacing between the text and line numbers what number to start at and the counting sequence.   When you’re done choosing your settings click OK.  Then Click OK again in the Page Setup dialog window.

That’s it!  You now have numbering added to the margins in your document.