Magento store config - get array

You cannot build this type of multi-dimensional arrays in the store config. If you want to store an array in the store_config you will need to serialize them using the backend-model for the field you want to store:

<backend_model>adminhtml/system_config_backend_serialized_array</backend_model>

With this backend you can build your own frontend model that extends the Mage_Adminhtml_Block_System_Config_Form_Field_Array_Abstract, here you can define your own fields you need (only text inputs can be created by default). When saving this will create an array that is stored serialized in the database, so when reading the value you will need to unserialize is:

$myvals = unserialize(Mage::getStoreConfig('myvals'));

UPDATE: If you do not need the backend input fields you can also save it yourself (and you should be able to use the multi-dimension):

Mage::getConfig()->saveConfig('myvals',serialize($myvals));