If you want to use a real Button, then you'll need to create a TemplateColumn, not an ActionColumn.
Drag a TemplateColumn over, then add a tpl (template). In the tpl definition, add this:
Code:
<tpl for="."><input type="button" value="ButtonText" onclick="myButtonOnClickFunction({recordField});"/></tpl>
Obviously you'll have to define some global myButtonOnClickFunction to handle the click event.
Now, if you want to use the ActionColumn....notice when you drag that over to your GridPanel it should create an ActionColumnItem object as well. So in there you set the icon class (iconCls) of the rendered "button". I suppose you could create your own graphic that looks like a button. You have to then add a handler to that ActionColumnItem for anything to happen when you click on it.
Hope this helps.