Hosted CanIt works fine with Google Apps. For the "route mail to" question when setting up a new domain, simply use the primary server which is likely to be something like "aspmx.l.google.com" Once the domain is set up, we can add the remaining ones later under Setup : Domain Routing.
Also, once the domain is set up, be sure to go to Preferences : Quarantine Settings and set S-930 "Enable SRS (Sender Rewriting Scheme)" to "Yes" in the default stream. Otherwise, Google might incorrectly flag a whole bunch of mail as failing SPF.
Please read Google's documentation on inbound gateways and set up Hosted CanIt to be your inbound gateway. The Hosted CanIt IP addresses are found under My Domains; be sure to include both the IPv6 and IPv4 networks as inbound gateways.