Not bad but, here you define the function as an int rather then a void yet it returns nothing. Why?
Code:
int MittSvar()
Code:
if (stykke == '+') svar = tall1 + tall2; // Here are we showing the Function how it should act when you put in the Values.
    if (stykke == '-') svar = tall1 - tall2; // Here are we showing the Function how it should act when you put in the Values.
    if (stykke == '*') svar = tall1 * tall2; // Here are we showing the Function how it should act when you put in the Values.
    if (stykke == '/') svar = tall1 / tall2; // Here are we showing the Function how it should act when you put in the Values.
Badly made since a switch/case is superior here.

Code:
system("PAUSE >nul");
Never use system calls for this, since you use C++ do cin.getline().

Otherwise great stuff.