Recently I had to use mail merge for a client without having outlook client installed.
Whenever I create a mail merge from web client, I don't seem to see the CRM add-ins under Add-ins tab. Then after doing some reaearch, I found that you need to have an entry in the host file, which is located at C:\Windows\System32\drivers\etc location on win 7 machine.
Just add an entry to your CRM IP address and name in a new line in this file.
For example, if your server IP is 192.168.1.1 and name is crmappserver, then add a new line with 192.168.1.1 and press tab and then enter crmappserver.
Save the file and open the CRM in IE. Create a mail merge template and now you should be able to see the CRM add-in button.