Create imageslider.xml file

Create the imageslider.xml file in app/design/adminhtml/default/default/layout/ 

<?xml version="1.0"?>
<layout version="1.0.0">
    <imageslider_adminhtml_imageslider_index>
        <reference name="content">
            <block type="imageslider/adminhtml_imageslider" name="imageslider" />
        </reference>
    </imageslider_adminhtml_imageslider_index>
</layout>

Note:- 1) imageslider_adminhtml_imageslider_index element represent the ModuleName_adminhtml_ControllerName_FunctionName
2) reference element which has name attribute and value is content.
in this block type imageslider/adminhtml_imageslider represents Modulename/adminhtml_BlockfileName.