I use Google Apps for my mail and the instructions there do not include how this should be setup for Site 5 unfortunately. I created two TXT records as such to get around this, but I cannot be sure that they are right?
Record Name: google._domainkey.mydomain.com.
TXT Data: v=DKIM1; k=rsa; p=MIG….
Record Name: mydomain.com.
TXT Data: v=spf1 include:_spf.google.com ~all
You can either have site5 do it for you: http://kb.site5.com/control/backstage/backstage-how-to-install-google-apps/
Or you just need to change the MX records for your domain.
Changing mx records: http://kb.site5.com/control/siteadmin/siteadmin-editing-mx-records/
Google apps MX record configuration: http://support.google.com/a/bin/answer.py?hl=en&answer=174125
The priority and destination values on the right side of the table are the ones you want.