Retrieving data from Html.DropDownList() in controller (ASP MVC) | string returned?

I have the following problem:

I have a form in site/banen (currently local running webserver) which is using a SQL database. The link is made using ADO.net and is instantiated in the controller in the following way:

DBModelEntities _entities;
_entities = new DBModelEntities(); // this part is in the constructor of the controller.

Next, I use this database to fill a Html.DropDownList() in my view. This is done in two steps. At the controller side we have in the constructor:

ViewData["EducationLevels"] = this.GetAllEducationLevels();

and a helper method:

public SelectList GetAllEducationLevels()
{
     List<EducationLevels> lstEducationLevels = _entities.EducationLevels.ToList();
     SelectList slist = new SelectList(lstEducationLevels, "ID", "Name");
     return slist;
}

In the view I have the following:

<% using (Html.BeginForm()) {%>

    <fieldset>
        <legend>Fields</legend>

        <!-- various textfields here -->

        <p>
            <label for="EducationLevels">EducationLevels:</label>
            <!-- <%= Html.DropDownList("EducationLevels", ViewData["EducationLevels"] as SelectList)%> -->
            <%= Html.DropDownList("EducationLevels", "..select option..")%>
        </p>
        <p>
            <input type="submit" value="Create" />
        </p>
    </fieldset>

<% } %>

Now, the form is rendered correctly when I browse to the create page. I can select etc. But when selected I have to use that value to save in my new model to upload to the database. This is where it goes wrong. I have the following code to do this in my controller:

//
    // POST: /Banen/Create

    [AcceptVerbs(HttpVerbs.Post)]
    public ActionResult Create(FormCollection form)
    {
        // set rest of information which has to be set automatically
        var vacatureToAdd = new Vacatures();
        //vacatureToAdd.EducationLevels = form["EducationLevels"];

        // Deserialize (Include white list!)
        TryUpdateModel(vacatureToAdd);

        // Validate
        if (String.IsNullOrEmpty(vacatureToAdd.Title))
            ModelState.AddModelError("Title", "Title is required!");
        if (String.IsNullOrEmpty(vacatureToAdd.Content))
            ModelState.AddModelError("Content", "Content is required!");

        // Update the variables not set in the form
        vacatureToAdd.CreatedAt = DateTime.Now;                 // Just created.
        vacatureToAdd.UpdatedAt = DateTime.Now;                 // Just created, so also modified now.
        vacatureToAdd.ViewCount = 0;                            // We have just created it, so no views
        vacatureToAdd.ID = GetGuid();                           // Generate uniqueidentifier

        try
        {
            // TODO: Add insert logic here
            _entities.AddToVacatures(vacatureToAdd);
            _entities.SaveChanges();

            // Return to listing page if succesful
            return RedirectToAction("Index");
        }
        catch (Exception e)
        {
            return View();
        }
    } 
    #endregion

It gives the error:

I have found various topics on this but all say you can retrieve by just using:

vacatureToAdd.EducationLevels = form["EducationLevels"];

Though this returns a string for me. Since I'm new to ASP.net I think I am forgetting to tell to select the object to return and not a string. Maybe this is the selectedValue in the part where I make my SelectList but I can't figure out how to set this correctly. Of course I can also be complete on a sidetrack.

Sidenote: currently I'm thinking about having a seperate model like here.

Any help is appreciated.