How do I add elements to an empty vector in a loop?

You need to use std::vector::push_back() instead:

while(cin >> x)
  myVector.push_back(x);
//         ^^^^^^^^^

and not std::vector::insert(), which, as you can see in the link, needs an iterator to indicate the position where you want to insert the element.

Also, as what @Joel has commented, you should remove the parentheses in your vector variable's definition.

std::vector<float> myVector;

and not

std::vector<float> myVector();

By doing the latter, you run into C++'s Most Vexing Parse problem.

If you want to use myVector.insert(), use it like myVector.insert(myVector.end(), x). This will append x at the end of myVector. You can insert x in the beginning by myVector.insert(myVector.begin(), x).

Another option is to use std::vector::emplace_back() instead of std::vector::push_back(). The makes some optimizations and doesn't take an argument of type vector::value_type, it takes variadic arguments that are forwarded to the constructor of the appended item, while push_back can make unnecessary copies or movements.

This is demonstrated in the std::vector::emplace_back documentation and here is a related question.

Usage example:

std::vector<int> myVector;

while (cin >> x) {
    myVector.emplace_back(x);
}

Use push_back:

while(cin >> x)
  myVector.push_back(x);

The insert function takes an iterator as the first argument, indicating the position to insert.

Also, you need to get rid of the parentheses in the declaration of myVector:

std::vector<float> myVector;