The mail merge feature in Word 2010 is much improved — this feature undergoes a big change in every version — and is easier to use than before. But it undoes an earlier fix: if your data source is an Excel file that contains formatted numbers and dates, these data will lose their formatting when they get placed into your document. For example, $10,000 will become 10000 and April 22, 2011 will become 4/22/11.
You can still edit the field codes in Word to manually insert formatting instructions (press Alt + F9 to toggle codes on and off and Ctrl + F9 to insert brackets), but it doesn’t always work properly and the codes can be inscrutable. I’ve found that the best solution is to use the Mail Merge Wizard (rather than simply choosing the data source manually) and connect to the Excel data using DDE.
Rather than go through all the steps here, download my PDF that has detailed instructions and screen shots. You can print it, if you want.