Magento – Source model not found for attribute “catcustomergroup”

i have added below code but i get above error:
Source model "unidesign/myoption" not found for attribute "catcustomergroup"

i have added custom field to category add i am using below installer script:

it works fine but i want to show customer group dropdown here , i am stucked u how can i do it

app\code\local\Devshree\Unidesing\etc\config.xml

<?xml version="1.0"?>
<config>
  <modules>
    <Devshree_Unidesing>
      <version>0.1.0</version>
    </Devshree_Unidesing>
  </modules>
  <global>

    <resources>
      <customcatattrb_setup>
        <setup>
          <module>Devshree_Unidesing</module>
          <class>Mage_Eav_Model_Entity_Setup</class>
        </setup>
        <connection>
          <use>default_setup</use>
        </connection>
      </customcatattrb_setup>
    </resources>
  </global>
</config> 

app\code\local\Devshree\Unidesing\sql\customcatattrb_setup\mysql4-install-0.0.1.php

<?php
$installer = $this;
$installer->startSetup();
$attribute  = array(
    'type' => 'select',
    'label'=> 'Customer Group',
    'input' => 'select',
    'global' => Mage_Catalog_Model_Resource_Eav_Attribute::SCOPE_GLOBAL,
    'visible' => true,
    'required' => false,
     'source' => "unidesign/myoption",
    'user_defined' => true,
    'default' => "",
    'group' => "General Information"
);
$installer->addAttribute('catalog_category', 'catcustomergroup', $attribute);
$installer->endSetup();
?>

app\code\local\Devshree\Unidesing\Model\Myoption.php

<?php
  Class MyNameSpace_Mymodule_Model_Myoption
 extends Mage_Eav_Model_Entity_Attribute_Source_Abstract
{
    /**
     * Retrieve all options array
     *
     * @return array
     */
   public function getAllOptions()
{
    if (is_null($this->_options)) {

     $options = Mage::getResourceModel('directory/country_collection')->load()->toOptionArray();
    array_unshift($options, array('value'=>'', 'label'=>Mage::helper('customer')->__('Select')));
        $this->_options =$options;
    }
    return $this->_options;
}
    /**
     * Retrieve option array
     *
     * @return array
     */
    public function getOptionArray()
    {
        $_options = array();
        foreach ($this->getAllOptions() as $option) {
            $_options[$option["value"]] = $option["label"];
        }
        return $_options;
    }

    /**
     * Get a text for option value
     *
     * @param string|integer $value
     * @return string
     */
    public function getOptionText($value)
    {
        $options = $this->getAllOptions();
        foreach ($options as $option) {
            if ($option["value"] == $value) {
                return $option["label"];
            }
        }
        return false;
    }

    /**
     * Retrieve Column(s) for Flat
     *
     * @return array
     */
    public function getFlatColums()
    {
        $columns = array();
        $columns[$this->getAttribute()->getAttributeCode()] = array(
            "type"      => "tinyint(1)",
            "unsigned"  => false,
            "is_null"   => true,
            "default"   => null,
            "extra"     => null
        );

        return $columns;
    }

    /**
     * Retrieve Indexes(s) for Flat
     *
     * @return array
     */
    public function getFlatIndexes()
    {
        $indexes = array();

        $index = "IDX_" . strtoupper($this->getAttribute()->getAttributeCode());
        $indexes[$index] = array(
            "type"      => "index",
            "fields"    => array($this->getAttribute()->getAttributeCode())
        );

        return $indexes;
    }

    /**
     * Retrieve Select For Flat Attribute update
     *
     * @param int $store
     * @return Varien_Db_Select|null
     */
    public function getFlatUpdateSelect($store)
    {
        return Mage::getResourceModel("eav/entity_attribute")
            ->getFlatUpdateSelect($this->getAttribute(), $store);
    }
}

problem i that i want to customer group drop down here how can i do this please help