Pages v5.2 does not have merge capability. If Pages ’09 v4.3 is still in your /Applications/iWork '09 folder, you can use that to manage a merge.
Or, if the addresses are in your Contacts, you can visit the File > Print page and choose Envelopes from the Style selector. The Layout tab allows you to choose between International, North American, Japanese, or custom envelopes. The individual tabs are reasonably self-explanatory. If you need further assistance, select the Contact > Help menu, and type envelope in the search field.