After upgrading to CRM 2013, we found out that mail merge button was missing in the command bar.
The solution was to customize the command for the existing mail merge button on the form.
For this open the solution containing the entity in Ribbon Work Bench and select Form of the Entity.
Right click the Mail Merge button and select Customize Command
Select commands and select Edit Display Rules
Remove Mscrm.HideOnCommandBar rule
Refresh the xml generated and publish the solution.
Hope it helps.