The support and development for this extension has been discontinued.
Make sure that you have enabled symlinks in your Magento installation. To enable this go to "System > Configuration > Advanced > Developer" and activate "Allow Symlinks".
Login trough SSH and go to the root of the Magento installation. Execute the following command:
cd .modman
git clone [email protected]:postnl/postnl-magento1.git
modman deploy postnl-magento1