You should just be able to add it as a tab under the tabs element, like in the XML below.
<?xml version="1.0" encoding="UTF-8"?>
<customUI onLoad="Ribbon_Load" xmlns="http://schemas.microsoft.com/office/2006/01/customui">
<ribbon>
<tabs>
<tab id="MyNewTab" label="My Tab" visible="true">
<group id="TabGroup" label="My Group" visible="true">
<button id="TestButton" onAction="TestButton_Click" label="Test Button" size="large" />
</group>
</tab>
</tabs>
</ribbon>
</customUI>
I'm not sure how you would do it from the designer but, you can access the Ribbon.xml file and add it that way.
Note that that is only for a custom tab. If you want to add it to a built in tab you need to specify it using the idMso of that tab. For example, if you wanted to add it to the appointment tab you could use:
<tab idMso="TabAppointment">
</tab>
Here is where you can find the control ids.