I want to add checkboxes in directory treeview to select multiple paths.

Use DataTemplate for TreeView. Before this, create a class for Folder(directory) with two properties,
1. FolderName - string property - to hold Folder's name.
2. CheckedStatus - bool property - to hold the selection status.
Add some more properties as per your requirement.
and now Folder class would be,

public class Folder
{
    public string FolderName { get; set; }
    public bool CheckedStatus { get; set; }
    //...............
    ...............//
}

And, define a DataTemplate in TreeView's XAML source like following,

<treeview name="FolderTree" margin="10,37,5,10">
   <treeview.resources>
      <datatemplate datatype="{x:Type local:Folder}">
         <stackpanel orientation="Horizontal">
            <checkbox ischecked="{Binding CheckedStatus, Mode=TwoWay}" />
            <image margin="4,0,0,0" width="15" height="15" source="Images/folder.png" />
            <textblock margin="4,0,0,0" text="{Binding FolderName}" />
         </stackpanel>
      </datatemplate>
   </treeview.resources>
</treeview>

Above code defines a DataTemplate for TreeView, the defined DataTemplate contains,
CheckBox - to allow users to select folders,
Image - to display folder image and
TextBlock - to show folder name.
These controls are wrapped into a StackPanel. Here Image is optional.

Now create an ObservableCollection to hold folders.

ObservableCollection<Folder> FolderCollection = new ObservableCollection<Folder>();

Add Folders of your specified location to this collection by creating objects for Folder Class.

foreach (DirectoryInfo dir in YourDirectoryObject.GetDirectories())
{
   Folder objFolder = new Folder();
   objFolder.FolderName = dir.Name;
   FolderCollection.Add(objFolder);
}

assign the prepared ObservableCollection FolderCollection to treeview.

FolderTree.ItemsSource = FolderCollection;

This will gives you a treeview with checkbox for selection. And you can loop through your FolderCollection to get selected Folders by checking CheckedStatus property.