How to make boxes align side-by-side instead of on top of each other in flexbox?

Wondering why the itemContainer and priceContainer won't show up next to each other?

Do I need to include any kind of file for FlexBox?

I thought it was built into CSS3. Is there a standard add-on to utilize?

.container {
  display: flex;
  display: -webkit-flex;
  width: 100%;
  height: 100%;
  flex-direction: row;
}
#orderContainer {
  width: 15%;
  border: 1px solid #f2f2f2;
  height: 100%;
  flex-direction: row;
}
#selectionsContainer {
  width: 85%;
}
#catagoryContainer {
  width: 100%;
  height: calc(20% - 2px);
  border: 1px solid #f2f2f2;
}
#menuContainer {
  width: 100%;
  height: 80%;
  border-top: 0px;
  border: 1px solid #f2f2f2;
}
#itemContainer {
  width: 70%;
  height: 80%;
  border: 1px solid #f2f2f2;
  display: flex;
  order: 1;
}
#priceContainer {
  width: calc(30% - 2px);
  height: 80%;
  border: 1px solid #f2f2f2;
  display: flex;
  order: 2;
}

<div class="container">
  <div id="orderContainer">
    <div id="itemContainer">
      itemContainer
    </div>
    <div id="priceContainer">
      priceContainer
    </div>
  </div>

  <div id="selectionsContainer">
    <section id="catagoryContainer">
      catagoryContainer
    </section>
    <section id="menuContainer">
      menuContainer
    </section>
  </div>
</div>