Hi truf and thank you for purchasing the game and for asking.
As hinted in the manual, the explosives have landed not very far from your starting point. Search for them in the areas located north and south of that point. Please remember that their location, like that of the partisans, changes every time you start a new game.
Once you find the explosives, you will automatically pick them up. To destroy a FlaK, defeat the artillery squad, then select the Attack icon.
Hope that helps!