You can try to give your item an ID and see if that helps:

Code:
{
	xtype: "TinyMCEEditor",
	itemId: 'MyMCE',
	fieldLabel: "Label",
	name: "testName",
	tinyMCESettings: { .. },
	width: 500,
	height: 300
},


var ed = form.down('#MyMCE');
Regards,
Scott.