1) First, configure the countries in the backend of your Shipping Manager .
Learn how to do this here.
Then, configure the countries in Magento, as follows:
2) In the Magento Admin Panel, go to System, then click on Configuration
3) Make sure the Configuration scope is set on Default Config
4) Then go to Sales, and click on Shipping Methods
5) Under bpost Home Delivery International and/or bpost Pick-up International - Options, Click on Import (countries). All international countries which you have defined in Shipping Manager for the specified delivery method will be imported.
6) Note that it is imperative to select the Allowed countries in Options, so you see them highlighted in blue (as seen below)
7) Save config