This Google knowledgebase article explains how to configure your MX records:  http://support.google.com/a/bin/answer.py?hl=en&answer=33915
You can change your MX records through cPanel > MX Entry

If you would like you can confirm your configuration step with following:

http://www.whatsmydns.net/#MX/

The following steps will help you to get your Google Mail MX records setup:

1. Log in to your cPanel account.
2. Click the MX Entry icon.
3. Under Domain, in the drop-down box, select the domain name you are changing.
4. Select the "Remote Mail Exchanger" option.
5. Click the Change button.
6. Paste the following combinations of Priorities and Destinations and add each one: (Be sure to remove the ending dot of each Destination so that cPanel will accept it.)
Priority: 1 Destination: ASPMX.L.GOOGLE.COM
Priority: 5 Destination: ALT1.ASPMX.L.GOOGLE.COM
Priority: 5 Destination: ALT2.ASPMX.L.GOOGLE.COM
Priority: 10 Destination: ASPMX2.GOOGLEMAIL.COM
Priority: 10 Destination: ASPMX3.GOOGLEMAIL.COM
Finally, delete the original MX record with Priority: 0 and Destination: yourdomain.com.