[Ethereum] Accessing not public variables

Main issue is you're calling a function in the foreign contract, so you use () instead of [].

Changed string to bytes32 so contracts can communicate. Also various touchups while I debugged it. bytes32 is more "native" and efficient, so it's generally best to do type conversion in the front-end.

I noticed the original concept was "ListProducts". Seems to be leading toward an indexed list of the mapped structs, so this might help: Are there well-solved and simple storage patterns for Solidity?

pragma solidity ^0.4.6;

contract Product{ 

  struct ProductStruct {  
    bytes32 name;  
    bool status;  
  }  

  mapping(uint => ProductStruct) public productStructs;

  function updateProduct(bytes32 name, uint ID) returns(bool success) {

  // following seems more like the intent. p[ID].n isn't valid

    productStructs[ID].name = name;
    productStructs[ID].status = true;
    return true;
  }
}

contract External {

  Product p;

  function External(address addr) {
    p = Product(addr);
  }

  function readProduct(uint u) constant returns(bytes32 name, bool status) {
    return(p.productStructs(u));
  }
}

Hope it helps.

This unit test works for me with the above contract using node 8.1.2 and truffle 3.4.3.

contract('Zusatzaufgabe', function(accounts) {
  it("Retrieve", async function() {
    const zusatzaufgabe = await Zusatzaufgabe.deployed();

    // Populate some entries
    await zusatzaufgabe.register("hola", "hola", 123, { from: accounts[1] });
    await zusatzaufgabe.register("toma", "joni", 321, { from: accounts[2] });

    // This returns Zusatzaufgabe.addressen[0]
    const addressen = await zusatzaufgabe.addressen.call(0);
    assert.equal(addressen, accounts[1]);

    // This returns Zusatzaufgabe.studenten[Zusatzaufgabe.addressen[0]]
    const student = await zusatzaufgabe.studenten.call(addressen);
    assert.equal(student[2].toNumber(), 123);
  });
});

The last call will return a tuple with the fields from struct Student. It will return [name, vorname, matnr, punkte]. The field solved is not returned, see https://solidity.readthedocs.io/en/develop/contracts.html#getter-functions for the documentation about getters automatically generated. Also int fieds (matnr and punkte) are returned as bignums.