how to implement binary tree in c#?

Andre is right to a certain extent, but his solution does not build you a full tree - he overwrites any existing node when you provide a different value.
If your tree values are meant to create a tree such as

    5
   / \
  2
 / \
1   3

Then you need to check is nodes exist already, before creating new ones.
In addition, I would not pass through the node to your add method: make it work on the current instance instead:

public class BTree
    {
    public int Value { get; set; }
    public BTree Left { get; set; }
    public BTree Right { get; set; }
    public BTree() : this(0) { }
    public BTree(int val)
        {
        Value = val;
        }
    public void Add(int val)
        {
        if (Value == 0)
            {
            Value = val;
            }
        else if (Value > val)
            {
            if (Left == null)
                {
                Left = new BTree(val);
                }
            else
                {
                Left.Add(val);
                }
            }
        else
            {
            if (Right == null)
                {
                Right = new BTree(val);
                }
            else
                {
                Right.Add(val);
                }
            }
        }
    }

Two other things occur:
1) Your Count is pretty useless: it returns the total number of nodes that have been cretaed, rather than the number which are in use, or which are involved in this tree. If you overwrite a node (as Andre does in his example), it does not decrease the count. You would be better off implementing a Count property which counted the number of nodes in teh tree from this point:

public int Count
    {
    get
        {
        int count = 1;
        if (Left != null) count += Left.Count;
        if (Right != null) count += Right.Count;
        return count;
        }
    }

2) You probably don't want a Print method - that word has a particular meaning in computing. Instead, consider providing a ToString method which returns the whole tree as a string:

public override string ToString()
     {
     StringBuilder sb = new StringBuilder();
     sb.Append(Value.ToString());
     if (Left != null || Right != null)
         {
         sb.Append(":(");
         if (Left != null)
             {
             sb.Append(Left.ToString());
             }
         sb.Append(",");
         if (Right != null)
             {
             sb.Append(Right.ToString());
             }
         sb.Append(")");
         }
     return sb.ToString();
     }

when creating a tree, the number of nodes in it changes as you add new elements, right?
so you need to create a new node for every 'add' operation

class BT
{
...
public void Add(BT t,int input)
{
...
else if(t.value>=input)
{
//add:  t.left = new BT();
t.left.value = input;           //<--- here t.left is null in your problem
t.left.left = null;
t.right.right = null;
count++;
}
else if(t.value<input)
{
//similarly here, 
//add: t.right = new BT();
t.right.value = input;
t.left.left = null;
t.right.right = null;
count++;
}
}
public void Print(BT tree)
{
Form1 o = new Form1();
if (tree.right != null)
Print(tree.right);
o.richTextBox1.AppendText(tree.value.ToString());
if (tree.left != null)
Print(tree.left);
}
}
}

Whenever you try to add a new node, you have to create it - you don't!
What you have to do is: When you add a new Left: you have to check if the left node exists, if it does, then you (presumably) want to add the new node it it, rather than the head. If it doesn't exist, then you can add a new node, and set the value.

It's difficult, because I am not sure what tree you expect to get from your inputs: try drawing yourself a tree of the expected results, and from that you should be able to work out where you need to add new nodes each time you add a value to the root node.