Customizing Ribbion Issue

I have the following code that does seems to work right:

<customUI xmlns="http://schemas.microsoft.com/office/2006/01/customui">
<ribbon startFromScratch="true">
<tabs>
<tab id="tabMain" label="CRSTHA Manager">
</tab>
</tabs>
</ribbon>
</customUI>

But when I try to add a button with the following code it doesn't work:
<customUI xmlns="http://schemas.microsoft.com/office/2006/01/customui">
<ribbon startFromScratch="true">
<tabs>
<tab id="tabMain" label="CRSTHA Manager">
<group id="grpHome" label="Home"/> <button id="cmdHome" label="Home" size="large" onAction="onOpenFormEdit" tag="frmMain"/>
</tab>
</tabs>
</ribbon>
</customUI>

Any ideas?