create a button in a new div:

var div = dojo.doc.createElement('div');
// append it to the body
dojo.body().appendChild(div);
// create a button
var button = new dijit.form.Button({ label:"foo" });
// append it to the div, addChild is a layout widget method, nothing to do with dom stuff:
// you want appendChild 
div.appendChild(button.domNode);
button.startup();

you can do it backwards, too by passing a node as a reference for your button

var div = dojo.doc.createElement('div');
dojo.body().appendChild(div);
var buttonNode = dojo.doc.createElement('div');
div.appendChild(buttonNode);
// now the widget stuff:
var button = new dijit.form.Button({ label: "foo" },buttonNode);
button.startup();

both should have the same results, though are different techniques based on how you want to manipulate the dom. I prefer to know where my nodes are and use the srcNodeRef (2rd param to new someDijit(...) ) over letting a widget create it's own node. either way, the node being used for the dijit can be accessed as .domNode always.