In your admin panel, under CONFIGURATION, you should have Google Merchant Centre Configuration showing in the dropdown.
You open that admin area, and you will see a host of options that need proper configuration. Do not miss ones you need, and do not use parameters that do not tally with Merchant Center's requirements (such as the Google Category).
BEFORE you generate the feed file (done from the Goggle Feed option under the TOOLS menu), you MUST get all your configuration settings correct.
The feed expiry, set as 29 days, we usually re-set to 28, meaning that there are four exact weeks between feeds. You cannot go beyond 30 days...
But this is a DUAL ACTIVITY... on the GOOGLE side you also need to get things properly configured - such as FTP username and password, which you then put into your zencart system...
Your very first feed should be a TEST FEED, so you can look at any errors Google throws back at you, and you can then re-tweak your zencart configuration settings to address those errors.
It can be a frustrating exercise, but once you have it right, its a breeze sending the feeds to Google.