Custom Block "Invalid Block Type"

As per as magento,the file/folder name after Block,Model,Helper folder should be start with Uppercase and only contain one uppercase for each folder and files.

According to magento the file name is Mymenu instead of MyMenu.

Class name should be MyNamespace_CustomerDashboard_Block_Mymenu

Your error says, it cant find the block MyNamespace_CustomerDashboard_Block_Mymenu. As per this hint, Your block class should be at app\code\local\MyNamespace\CustomerDashboard\Block\Mymenu.php. Please note you have MyMenu.php. It should be Mymenu.php. Also you need to ensure the class name is MyNamespace_CustomerDashboard_Block_Mymenu.

or

you can avoid this error by simply changing your local.xml update. It should look like this.

<reference name="left">
    <block type="customerdashboard/myMenu" name="customerdashboard.mymenu" template="customer/form/my-menu.phtml"/>
</reference>

See the type now we are using. It is customerdashboard/myMenu instead of customerdashboard/mymenu. Now this type will refer to the block class MyNamespace_CustomerDashboard_Block_MyMenu. This way your block will be recongnized by Magento and problem will resolve

Note : Dont forget to clear the cache.