Magento – Admin Dashboard – Add JavaScript

Instead of overwriting the page.xml I would suggest using the layout/local.xml file in your active template.

If it doesn't exist please create the file app/design/frontend/default/blank/layout/local.xml and add the page.xml content you've already created.

Please make sure caching and JS script compression are disabled to see the changes

This should be adminhtml below routers and not configsliders in your config.xml.

<admin>
    <routers>
        <adminhtml>                               
            <args>
                <modules>
                    <Namespace_Configslider after="Mage_Adminhtml">Namespace_Configslider_Adminhtml</Namespace_Configslider>
                </modules>                                
            </args>
        </adminhtml>
   </routers>
</admin>

Also, I'd recommend this guide.

http://alanstorm.com/magento_admin_hello_world_revisited

Updated answer based on new info.

Without re-creating the entire module, this is what I changed to get the admin page to show up and not 404. I substituted the namespace for my testing so you will need to change that. You should be able to take it from here.

Pay close attention to the order of the namespace_extensionname in the config edits, these are important.

Modified: app/etc/local/Stackexchange/Configslider/etc/config.xml

<?xml version="1.0" encoding="UTF-8"?>
<config>
    <modules>
        <Stackexchange_Configslider>
            <version>0.1.0</version>
        </Stackexchange_Configslider>
    </modules>

    <global>
        <helpers>
            <stackexchange_configslider>
                <class>Stackexchange_Configslider_Helper</class>
            </stackexchange_configslider>
        </helpers>

        <blocks>
            <configslider>
                <class>Namespace_Configslider_Adminhtml_Block</class>
            </configslider>
        </blocks>
    </global>

    <admin>
        <routers>
            <adminhtml>
                <args>
                    <modules>
                        <Stackexchange_Configslider after="Mage_Adminhtml">Stackexchange_Configslider_Adminhtml</Stackexchange_Configslider>
                    </modules>
                </args>
            </adminhtml>
        </routers>
    </admin>

    <adminhtml>
        <layout>
            <updates>
                <stackexchange_configslider module="stackexchange_configslider">
                    <file>stackexchange_configslider.xml</file>
                </stackexchange_configslider>
            </updates>
        </layout>
    </adminhtml>
</config>

Modified: app/etc/local/Stackexchange/Configslider/etc/adminhtml.xml

<?xml version="1.0"?>
<config>
    <menu>
        <cms>
            <children>
                <stackexchange translate="title" module="stackexchange_configslider">
                    <title>Configure the slider</title>
                    <sort_order>50</sort_order>
                    <action>adminhtml/configslider/index</action>
                </stackexchange>
            </children>
        </cms>
    </menu>

    <acl>
        <resources>
            <admin>
                <children>
                    <cms>
                        <children>
                            <stackexchange translate="title" module="stackexchange_configslider">
                                <title>Configure the slider</title>
                                <sort_order>100</sort_order>
                            </stackexchange>
                        </children>
                    </cms>
                </children>
            </admin>
        </resources>
    </acl>
</config>

Missing: app/etc/local/Stackexchange/Configslider/controllers/Adminhtml/ConfigsliderController.php

<?php

class Stackexchange_Configslider_Adminhtml_ConfigsliderController extends Mage_Adminhtml_Controller_Action
{
    public function indexAction()
    {
        $this->loadLayout();
        $this->renderLayout();
    }
}