Cirrus Insight does not require any internal customization to working with custom objects. When custom objects are created in Salesforce and are accessible to the current user, they will appear automatically in Cirrus Insight if they are related to the Account or Contact objects. All attributes of the object will act exactly as configured in Salesforce.
The custom object should appear in the "Create New" and Related List areas just like a standard object. Note that the Salesforce configuration of the custom object must allow activities for Cirrus Insight to be able to store emails, log calls, or create other activity records.
If a custom object is not visible in Cirrus Insight, the most common reason is that it is not accessible to the Salesforce profile of the user. In rare cases, a Cirrus Insight admin may request that support suppress objects from the Cirrus Insight + or related lists.